Auto-generate preview links for PRs

[joepeeples]

Description

We can implement a GitHub Action to automatically generate preview links for our PRs. Several other docs teams/repos already do this, so we can use their solutions as inspiration:

• Enterprise Search - PR to add action
• Logstash - action YAML

Acceptance Test Criteria

Preview links are generated when a PR is created, and updated when PR has new changes merged in.

Notes

• This isn't exactly version-specific, but added v8.5.0 label as a general window when we want to roll this out (definitely wait until after 8.4.0 release).
• Auto-generated links will probably just be a top-level link to the main landing page, so there will still be value in manually adding links to target only the changed pages. Investigate if we could add any magic to generate targeted links? ✨

Additional tasks

Reopened this issue to complete a few more tasks, now that we know the action works:

• Backport to rest of 8.x series and 7.17 (use backport tool)
• Update readme in security-docs to explain how auto-previews work (and backport that too: create new PR and label accordingly so Mergify creates backports)

[joepeeples]

Reopening to complete a few more tasks, now that we know the action works:

• Backport to rest of 8.x series and 7.17
• Update readme in security-docs to explain how auto-previews work (and backport that too)

[joepeeples]

Closing (again); GitHub Action and readme update backported to 7.17+.