Add developer documentation about Banzai debugging
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=422527) </details> <!--IssueSummary end--> ### Proposal Discussion: https://gitlab.com/gitlab-org/gitlab/-/issues/382709#note_1509239660 Add a developer documentation to explain how to properly debug performance issues coming from Banzai markdown rendering. * Explain how to use flamegraphs * Caveats (Banzai caching) * Hints and tips
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