Add docs CI workflow

[mattheliu]

Summary

• add a dedicated GitHub Actions workflow for documentation changes
• run markdown linting with pymarkdown
• build the MkDocs site with --strict to catch broken nav, missing pages, and build warnings

Scope

• triggers only for docs/**, .readthedocs.yaml, and the docs workflow file
• does not change existing code lint or GPU test workflows

Local validation

• uv sync --extra lint
• uv run pymarkdown scan source/
• uv run mkdocs build --strict

[paddle-bot]

Thanks for your contribution!