Document the Memory Report

In #350152 (closed) (!78670 (merged)) we moved the Memory Report to the performance bar. We should properly document this feature, how to use it, when to use it, etc.

We have this as a broader goal in the epic, see #349958 (closed)

Existing documentation: https://docs.gitlab.com/ee/development/performance.html#using-memory-profiler

https://docs.gitlab.com/ee/development/cached_queries.html#how-to-measure-the-impact-of-the-change

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Related to #349958 (closed).

Related to #350152 (closed).