Add `n_best_runs` param detail in mm nb

[iamrk04]

Description

This PR adds detail about the n_best_runs parameter in the Many Models v2 notebook. It allows users to track n child runs per partition in a Many Model run.

Checklist

• [x] I have read the contribution guidelines.
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• [x] I have coordinated with the docs team (mldocs@microsoft.com) if this PR deletes files or changes any file names or file extensions.
• [x] Pull request includes test coverage for the included changes.
• [x] This notebook or file is added to the CODEOWNERS file, pointing to the author or the author's team.