Fix documentation publishing workflow

[nogates]

Summary

• minify generated package and source HTML before publishing
• publish documentation through GitHub Pages artifacts instead of committing the output to gh-pages
• restrict the build to read-only repository access and grant Pages permissions only to the deployment job

This fixes the documentation failure exposed by #4668, where the generated datadogV2/index.html exceeded GitHub’s 100 MiB Git blob limit.

Verification

• pre-commit run --files .github/workflows/docs.yml
• actionlint .github/workflows/docs.yml
• reproduced the Publish stable Experiments v2 APIs #4668 merge build: the largest page is 103,436,024 bytes after minification and the complete Pages artifact is approximately 293 MiB

Rollout note

After this merges, change Settings → Pages → Build and deployment → Source from the legacy gh-pages branch to GitHub Actions before rerunning the documentation deployment.