docs: Add missing link to schema for `merge_queue` repository rule

[usmonster]

Resolves #2579

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Before the change?

• merge_queue ruleset docs missing link to rule schema.
• merge_queue.max_entries_to_merge rule description is copy-pasted from merge_queue.max_entries_to_build.

After the change?

• The link to the rule schema has been added.
• merge_queue.max_entries_to_merge rule description is correct.

Pull request checklist

• [ ] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• [ ] Yes
• [x] No

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[usmonster]

Hello! Can this please be reviewed (& hopefully merged) by a maintainer? Thanks! (cc: @kfcampbell)