doc: Debugging Gitaly

Why is this change being made?

Primarily aimed at Create oncall training for Gitaly (gitlab-org/core-platform-section/discussions#178), also serve as a basis for debugging customer issues.

This is NOT oncall training, but a supporting document for that, as well as for customer issues or just simply debugging while developing.

The main goal is to bring everyone on the same page about what tools are available, and lower their barrier of entry. It's intended both for newcomers and as reference material in day-to-day.

Author and Reviewer Checklist

Please verify the check list and ensure to tick them off before the MR is merged.

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Commits

• doc: Debugging Gitaly

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