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 🙇