[CCT-1506] Feature documentation for custom forecast

[dmytro-kliagin-dd]

What does this PR do? What is the motivation?

Documents the custom forecast feature for Cloud Cost Management budgets:

• Budgets page: Adds a "Customize your budget forecast" section covering the Bits forecast vs. custom forecast vs. final forecast concepts, how to add or edit custom forecast values when creating or editing a budget, the forecast cell states, and how custom forecasts are used in budget status, monitors, CSV export, and on delete.
• Forecasting page: Adds a "Bits and custom forecasts" concept section and a cross-link from "View forecasts in budgets" to the Budgets how-to.

Scope is limited to the V1 release. Fast-follow items (view toggle between Bits and final forecast, budget reports integration, Terraform, CSV bulk upload, edit history) are intentionally not documented.

Merge instructions

Merge readiness:

• Ready for merge

Additional notes

Holding merge until the feature is generally available.

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/dmytro.kliagin/CCT1506-custom-forecast-docs/cloud_cost_management/planning/budgets
• https://docs-staging.datadoghq.com/dmytro.kliagin/CCT1506-custom-forecast-docs/cloud_cost_management/planning/forecasting

[drichards-87]

Left some feedback from Docs.

[drichards-87]

Suggested change

	The forecast that Datadog generates automatically from your historical spend is called the **Bits forecast**. Because it is trend-based, it projects past patterns forward but cannot account for business events you are planning, such as a product launch, a migration, or seasonal demand.
	The forecast that Datadog generates automatically from your historical spend is called the **Bits forecast**. Because it is trend-based, it projects past patterns forward but cannot account for planned business events, such as a product launch, migration, or seasonal demand.

[drichards-87]

Suggested change

	In [budgets][3], you can override the Bits forecast with your own monthly values, called a **custom forecast**. Datadog will overlay your custom forecast on top of the Bits forecast, so your overrides take precedence wherever you set them. The forecast with your overrides is what budgets and budget monitors use by default.
	In [budgets][3], you can override the Bits forecast with your own monthly values, called a **custom forecast**. Datadog overlays your custom forecast on the Bits forecast, so your overrides take precedence wherever you set them. Budgets and budget monitors use the forecast with your overrides by default.

[drichards-87]

Suggested change

	By default, Datadog combines the automatic Bits forecast with any custom forecast values you set in the budgets. To override the Bits forecast with your own monthly values, see [Customize your budget forecast][4].
	By default, Datadog combines the automatic Bits forecast with any custom forecast values you set in budgets. To override the Bits forecast with your own monthly values, see [Customize your budget forecast][4].

[drichards-87]

Suggested change

	- **Budget status**: The budget status page and the forecast totals on the Budgets page include your custom forecast.
	- **Budget monitors**: [Budget monitors][2] when evaluating, a custom forecast takes precedence over the Bits forecast when one is present.
	- **CSV export**: Downloading a budget as CSV includes the custom forecast values where it was set.
	- **Deleting a budget**: Deleting a budget also deletes its associated custom forecast values.
	- **Budget status**: The budget status page and the forecast totals on the Budgets page include your custom forecast.
	- **Budget monitors**: When [budget monitors][2] evaluate, a custom forecast takes precedence over the Bits forecast when one is present.
	- **CSV export**: Downloading a budget as a CSV includes the custom forecast values where they are set.
	- **Deleting a budget**: Deleting a budget also deletes its associated custom forecast values.

[drichards-87]

Suggested change

	<div class="alert alert-info">Datadog saves the budget first, and then the custom forecast. If the budget saves but the custom forecast does not, a notification prompts you to retry from the edit page.</div>
	<div class="alert alert-info">Datadog saves the budget first, and then saves the custom forecast. If the budget saves but the custom forecast does not, a notification prompts you to retry from the edit page.</div>

[drichards-87]

Suggested change

	While editing, the appearance of a forecast cell indicates its state:
	While editing, the appearance of each forecast cell indicates its state:

[drichards-87]

Suggested change

	1. On the [Budgets page](https://app.datadoghq.com/cost/plan/budgets), click the edit icon for a budget.
	1. The forecast columns appear automatically when you have the `ccm_forecast_write` permission. Each forecast cell displays your saved override, or the Bits forecast as a gray placeholder when no override exists.
	1. Enter or change a dollar amount in any forecast cell. Negative values are not allowed.
	1. To compare your overrides against the original automatic values, toggle {{< ui >}}Show Bits AI forecast{{< /ui >}} to display a read-only Bits AI column next to each forecast column.
	1. Click {{< ui >}}Save{{< /ui >}}.
	1. On the [Budgets page][1], click the edit icon for a budget.
	The forecast columns appear automatically if you have the `ccm_forecast_write` permission. Each forecast cell displays your saved override, or the Bits forecast as a gray placeholder when no override exists.
	1. Enter or change a dollar amount in any forecast cell. Negative values are not allowed.
	1. To compare your overrides against the original automatic values, toggle {{< ui >}}Show Bits AI forecast{{< /ui >}} to display a read-only Bits AI column next to each forecast column.
	1. Click {{< ui >}}Save{{< /ui >}}.
	[1]: https://app.datadoghq.com/cost/plan/budgets

[drichards-87]

Suggested change

	1. Each forecast cell displays the Bits forecast as a gray placeholder. Enter a dollar amount to override it. Negative values are not allowed.
	{{< img src="cloud_cost/budgets/cust-fcst-during-create-table.png" alt="Toggle Customize Bits Forecast to display forecast columns" style="width:100%;">}}
	1. The preview chart updates as you edit, so you can review the final forecast before saving.
	1. Each forecast cell displays the Bits forecast as a gray placeholder. Enter a dollar amount to override it. Negative values are not allowed.
	The preview chart updates as you edit, so you can review the final forecast before saving.
	{{< img src="cloud_cost/budgets/cust-fcst-during-create-table.png" alt="Toggle Customize Bits Forecast to display forecast columns" style="width:100%;">}}

[drichards-87]

Suggested change

	Datadog automatically generates a **Bits forecast** for each budget, projecting future costs from your historical spend. When you have knowledge that the automatic forecast cannot capture, such as a planned product launch, a migration, seasonal demand, or workloads being retired, you can override the Bits forecast with your own values. This override is called a **custom forecast**.
	Datadog automatically generates a **Bits forecast** for each budget, projecting future costs from your historical spend. When you have knowledge that the Bits forecast cannot capture, such as a planned product launch, migration, seasonal demand, or retired workloads, you can override it with your own values. This override is called a **custom forecast**.