[DOCS]: Add missing link to schema for merge_queue repository rule

[usmonster]

Describe the need

Merge queue support was added to repository rulesets in #2380, but the docs are missing a link to the rule schema.

Let's add it for consistency and to make the docs easier to use.

SDK Version

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API Version

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[zepeng811]

I assume you're mentioning something like "(see below for nested schema)" which other attributes have.

I also noticed an incorrect description for max_entries_to_merge (it's the same description as max_entries_to_build), can probably fix it together in one PR

[usmonster]

Thanks @zepeng811, a PR is open at #2581.