Add GitHub Actions workflow to auto-publish docs

[sachinnn99]

Summary

• Adds a GitHub Actions workflow (.github/workflows/docs-publish.yml) that automatically builds and deploys MkDocs documentation to GitHub Pages on pushes to master that modify docs/.
• Uses mkdocs gh-deploy --force — the same command currently run manually.
• Includes concurrency control to prevent race conditions on simultaneous deploys.

Closes #358

Details

• Triggers only on docs/** path changes to avoid unnecessary runs
• Uses github-actions[bot] identity for gh-pages commits
• Installs the same 3 dependencies listed in docs/README.md: mkdocs, mkdocs-material, pymdown-extensions
• Follows the official mkdocs-material publishing guide

Test plan

• Verify workflow YAML syntax is valid
• After merge, push a docs change to master and confirm the Actions tab shows the workflow running
• Confirm https://netflix.github.io/mantis updates with the change