Document branch filters for merge_group workflows

[alan506302-lang]

Adds a dedicated workflow syntax section for on.merge_group.<branches|branches-ignore>.

The new section documents:

• using branches to include target branch patterns
• using branches-ignore to exclude target branch patterns
• glob pattern behavior
• examples for both filters

Resolves #32879

[github-actions]

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Source	Review	Production	What Changed
actions/reference/workflows-and-actions/workflow-syntax.md	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17

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[alan506302-lang]

Thanks for reviewing! All checks are passing. This PR addresses the help wanted issue by adding a dedicated merge_group branch filter section as requested.