ci: standardize docs workflow

[stefan-jansen]

Summary

• standardize the library docs deployment workflow on the shared website handoff pattern
• pin astral-sh/setup-uv to the official v8.1.0 commit from Astral's setup-uv documentation
• build docs with uv run mkdocs build and deploy site/ to website/static/docs/engineer/ when the deploy key is present

Notes

• preserves the existing website-side docs rendering and navbar injection contract
• keeps the deploy target and URL structure unchanged

[Copilot]

Pull request overview

Standardizes the documentation build/deploy GitHub Actions workflow to match the shared “website handoff” pattern, including pinning setup-uv and conditionally deploying generated MkDocs output to the website repository.

Changes:

• Pins astral-sh/setup-uv to a specific commit (v8.1.0) for the docs workflow.
• Builds docs via uv run mkdocs build and conditionally deploys site/ when a deploy key is present.
• Routes deploy key through a job-level env var and uses it to gate the deploy step.

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[stefan-jansen]

Superseded by the later GitHub Actions/Dependabot maintenance PR that has already been merged to main.