Add PR Builds and Previews

[bmorelli25]

Summary

Documentation PRs should need CI that validates the content before it can be merged.

When a PR is opened or Synchronized, our Asciidoc PR builds do the following.:

• Build the PR on BuildKite
• Check all internal links
• Push the built HTML files to the elastic/built-docs directory
• Publish built HTML files so the entire content set is viewable in the context of the PR
• Show a diff of the built HTML files for easy validation

Requirements

To be scoped

[bmorelli25]

• Can potentially reuse vercel publish action in the meantime
• Can potentially spin up an S3 bucket, create an S3 website and have the action post to S3
  • Use object retention & know when PRs are merged
  • Don't have to handle credentials, auth handled by gh repo access to aws