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

### 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

_No response_

### API Version

_No response_

### Relevant log output

```shell

```

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct