Auto-generate preview links for PRs

[joepeeples]

Addresses #2312. Adds a GitHub Action that will automatically create preview links when a pull request is created. Links are added in a comment on the PR.

Notes

• The GitHub Action runs as a check alongside other checks (such as CI, Mergify, etc.) but only runs when the PR is first opened.
• Heavily modeled after the GH Action that Enterprise Search recently implemented in https://github.com/elastic/enterprise-search-pubs/pull/2666.
• Future enhancement possibilities:
  • Wait until CI has successfully built the preview, to ensure no dead links (Logstash's preview Action currently does this).
  • Create targeted links to the specific pages changed in the PR (currently the docs link is just top-level landing page).

[bmorelli25]

Looks pretty good to me 🚀

[joepeeples]

Some teams are using path filters to create preview links only when PRs involve certain paths (such as docs/*) or file types (such as .asciidoc), but that doesn't seem necessary for the security-docs repo.

This repo is self-contained to focus only on docs, so we don't need to account for non-docs PRs the way other teams might if their docs cohabitate with code. We can always add filters later if we find the Action to be too noisy, but I think we'll want preview links for the vast majority of PRs in security-docs.

[jmikell821]

This LGTM! 👍 Thanks for getting this set up :)

[nastasha-solomon]

🚀 🚀 🚀

[joepeeples]

💚 All backports created successfully

Status	Branch	Result
✅	8.3
✅	8.2
✅	8.1
✅	8.0
✅	7.17

Questions ?

Please refer to the Backport tool documentation