No doc of `on.merge_group.<branches|branches-ignore>`

### Code of Conduct

- [X] I have read and agree to the GitHub Docs project's [Code of Conduct](https://github.com/github/docs/blob/main/.github/CODE_OF_CONDUCT.md)

### What article on docs.github.com is affected?

https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions

### What part(s) of the article would you like to see updated?

GitHub Actions `merge_group` trigger supports `branches` filter since its [beta release](https://github.blog/changelog/2022-08-18-merge-group-webhook-event-and-github-actions-workflow-trigger/) in Aug 2022, but it's not documented till now.

I guess the symmetric `branches-ignore` filter is also supported but didn't check it myself.

One option is to extend current section [`on..`](https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#onpull_requestpull_request_targetbranchesbranches-ignore) to cover `merge_group` and taking care of old anchor name redirection. Or add a separate `on.merge_group.` section.

### Additional information

Relevant GitHub Blogs

- Beta https://github.blog/changelog/2022-08-18-merge-group-webhook-event-and-github-actions-workflow-trigger/
- Public beta https://github.blog/changelog/2023-04-19-pull-request-merge-queue-public-beta-api-support-and-recent-fixes/
- Generally available https://github.blog/changelog/2023-07-12-pull-request-merge-queue-is-now-generally-available/

Related
- https://github.com/actions/languageservices/pull/12
- https://github.com/actions/languageservices/issues/18
- Path filtering does not work for merge_group triggers https://github.com/orgs/community/discussions/45899