Document on.merge_group branches and branches-ignore filters (#32879)

[sudorm-rf0]

Resolves #32879 by documenting the branches and branches-ignore filters for the merge_group event.

Both filters are supported for merge_group — the GitHub Actions team confirmed branches-ignore support directly in the issue thread (validated with a branches-ignore: ["main"] workflow). branches has been supported since the merge queue beta (Aug 2022).

This adds a short note plus two YAML examples (include and exclude) to the end of the ## merge_group section of events-that-trigger-workflows.md, mirroring the existing branches/branches-ignore documentation style used for pull_request.

Note: this is an English-only content addition; localized files remain behind English, which is the allowed direction for the localization sync check.

[github-actions]

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Table of review links

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The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
actions/reference/workflows-and-actions/events-that-trigger-workflows.md	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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[subatoi]

Thank you, @sudorm-rf0! There's another PR open for this issue, so I'll try to resolve the issue to as many people's benefit as possible. Unfortunately I can't guarantee a timeframe for a response, but we appreciate your interest in the docs 🙇

[github-actions]

A stale label has been added to this pull request because it has been open 30 days with no activity. If you think this pull request should remain open, please add a new comment.

[subatoi]

Hello again—unfortunately we've gone ahead with #43500, as it was raised before this, and was more comprehensive per the agreed contribution guidelines. Sorry this won't have been the answer you were looking for, and thank you for your interest in the GitHub docs.