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