Document AI Obs End-user Feedback Feature

[mtullalizardi]

What does this PR do?

This PR adds customer-facing docs for the new end-user feedback feature in Agent Observability.

It adds a new End-User Feedback page under Evaluations, updates the Evaluations overview and navigation, and expands the HTTP API reference to document event_kind: feedback, submitter, text_value, and feedback_join_key.

Motivation

Customers need docs for submitting thumbs-up/down ratings, accepted changes, free-text comments, and other feedback signals without treating them as normal external evaluations.

This follows the user feedback RFC: https://docs.google.com/document/d/1OpFg6L5dj3yleyClRb1yRyG1QP93Ma3Pf6gNlroqsOc/edit

Source Code PRs

• Data model: https://github.com/DataDog/dd-source/pull/442890
• Standalone feedback event processing: https://github.com/DataDog/dd-source/pull/443018
• Feedback eval-metric intake: https://github.com/DataDog/dd-source/pull/444661
• Feedback eval-metric validation: https://github.com/DataDog/dd-source/pull/447413
• Release Agent feedback instrumentation: https://github.com/DataDog/dd-source/pull/452124
• feedback_join_key as a first-class span attribute: https://github.com/DataDog/dd-source/pull/458526

Important Details

The docs call out the current limitation that filtering spans, traces, or sessions by feedback is not available yet. Dashboard widgets should be created like evaluation widgets and filtered with @event_kind:feedback.

These features will land soon and before this feature is GA.

AI Assistance

This PR was written with AI assistance and reviewed locally before opening.

QA

https://docs-staging.datadoghq.com/mtl/end-user-feedback-docs/llm_observability/evaluations/end_user_feedback

[mtullalizardi]

DOCS-14675 for the editorial review.

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/mtl/end-user-feedback-docs/llm_observability/evaluations/end_user_feedback

Modified Files

• https://docs-staging.datadoghq.com/mtl/end-user-feedback-docs/llm_observability/evaluations/
• https://docs-staging.datadoghq.com/mtl/end-user-feedback-docs/llm_observability/evaluations/external_evaluations
• https://docs-staging.datadoghq.com/mtl/end-user-feedback-docs/llm_observability/instrumentation/api

[cswatt]

Made some small language edits, most notably to the info box about support for filtering spans/traces/sessions, just to bring everything in line with styleguide. Otherwise everything looks good!