Add Linear integration documentation

[mintlify]

Summary

Adds a new documentation page for the Mintlify + Linear agent integration (agent/linear.mdx) and adds it to the navigation in docs.json.

The page covers:

• Connecting a Linear workspace to the Mintlify agent
• Using @mintlify mentions in Linear issues and comments to trigger documentation updates
• Example prompts and best practices

Follows the same pattern as the existing Slack integration page (agent/slack.mdx).

Notes for reviewers

• This page includes a TODO comment for a screenshot of the Linear integration settings, which should be added once available.
• Since this is a new product feature, please verify the setup flow and feature details match the actual product behavior.

Resolves DOC-306

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Note

Low Risk
Low risk: documentation-only updates and navigation wiring; main risk is user confusion if the described Slack/Linear install flows or screenshots don’t match the current dashboard UI.

Overview
Adds new agent/linear.mdx documentation describing how to install the Linear app, trigger doc updates via @mintlify mentions/assignment, and follow best practices.

Updates the agent landing page and docs.json navigation to surface the Linear integration, and revises existing Slack + automation guide instructions to match the new “Agent settings” install flow/screenshots.

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