chore: update docs writer agent to build mkdocs

[aviatco]

📥 Pull Request

✨ Description of new changes

This pull request updates the documentation workflow in .github/agents/docs-writer.agent.md to include a required local build and preview step before delivering documentation changes. The new instructions ensure documentation edits are validated for broken links, navigation issues, and formatting errors prior to delivery.

Documentation workflow improvements:

• Added a new "Build and preview locally" section, instructing contributors to install MkDocs with the Material theme and section index plugin, and to run mkdocs build to validate documentation changes before delivery. This helps catch issues early and ensures the site builds cleanly.
• Clarified that mkdocs serve should not be run as part of the validation step, but should be used by the person updating the docs for interactive previewing.
• Updated the delivery checklist to require confirmation that the site builds cleanly and to provide instructions for previewing the documentation locally.

[Copilot]

Pull request overview

Updates the docs-writer agent workflow guidance to require a local MkDocs build validation step before delivering documentation changes, helping catch broken links/nav/config issues earlier.

Changes:

• Added a new “Build and preview locally” step with MkDocs install/build instructions.
• Clarified that mkdocs build is the validation step and mkdocs serve is for interactive preview by the docs author.
• Updated the delivery checklist to require confirming a clean MkDocs build and providing preview instructions.