github-action: Add AsciiDoc freeze warning

[reakaleek]

Details

⚠️ This PR was created by an automated tool. Please review the changes carefully. ⚠️

Add a workflow that will comment on PRs with AsciiDoc changes.

Why

During the migration to Elastic Docs v3, the Docs team will focus exclusively on migrating content.
To maintain consistency, prevent conflicts, and ensure a smoother transition we will freeze all AsciiDoc changes.

This means you will get a warning when you create AsciiDoc changes in your PRs.

See elastic/docs-builder#281 for details

If there are any questions, please reach out to the @elastic/docs-engineering

[mergify]

This pull request does not have a backport label. Could you fix it @reakaleek? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• backport-8.x is the label to automatically backport to the 8.x branch.

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label.