docs: document branch filters for merge_group workflows

[hiromieguchi802-lab]

Summary

• document �ranches and �ranches-ignore for merge_group in workflow syntax
• add matching guidance to the merge_group event docs
• update the example workflow to show filtering merge groups that target main

Fixes #32879.

Testing

• reviewed the diff for the two updated docs pages
• attempted to run lint-content, but the local environment is missing or has broken dependencies ( sx was initially unavailable, then highlight.js/lib/languages/golo could not be resolved)

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Source	Review	Production	What Changed
actions/reference/workflows-and-actions/events-that-trigger-workflows.md	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15 3.14
actions/reference/workflows-and-actions/workflow-syntax.md	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15 3.14

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