github-action: add supported GitHub commands

[v1v]

Details

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

Explicitly explain what GitHub commands are supported.

Why

We want to improve our user experience and make it easier for those GitHub commands
that are supported by the Elastic CI Ecosystem.

It uses https://github.com/elastic/oblt-actions/tree/main/elastic/github-commands

If there are any questions, please reach out to the @elastic/observablt-ci

[mergify]

This pull request does not have a backport label. Could you fix it @v1v? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.
  NOTE: backport-skip has been added to this pull request.

[dliappis]

LGTM