Auditbeat: Add placeholder for new docs directory and file

[karenzone]

We need a placeholder for accurate testing and to break docs-ci gridlock between repos

• Doc: Add docs for Auditbeat add_session_metadata processor #40252 won't pass docs-ci because doc build is not pulling from the new repo.
• Add x-pack-auditbeat to conf.yaml docs#3038 won't pass docs-ci because the files we're trying to pull in haven't been merged.

[botelastic]

This pull request doesn't have a Team:<team> label.

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @karenzone? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[karenzone]

@mjwolf will you please review/approve so that I can merge elastic/docs#3038. That PR is required to break the gridlock so that I can get accurate docs-ci checking on #40252.

[karenzone]

Need code owner review to unblock this work, please. (See description for details.)
Let me know if you have concerns.

[karenzone]

11816/7