Create technical documentation describing the several components used to render a MR Diff

Context

In the context of the newly formed Performance Round TablesCode Review, we noticed some knowledge gaps. Some of the team has partial knowledge of the entire system (namely across the backend/frontend divide).

Having a better notion of the entire system could lead to identifying significant improvements.

Proposal

• backend Document the backend process of calculating and serving the diffs information to the frontend
• frontend Document the frontend process of rendering the Merge Request diffs with the information provided by the backend.