New Diffs: Document current implementation of Diffs across GitLab

Context

In the context of the New Diffs effort (Epic), we aim to use this issue to collaborate documenting the current implementation of Diffs.

Scope

Implementations to document:

• Repository Commit
• Compare Branches
• Merge Requests (Changes)
• New Merge Request form
• ...?

Note: We'll need to document how data flows from gitaly to rails to caching to frontend.

How?

We should keep it somewhat simple but perhaps a mermaid chart with a blurb of text explaining details could suffice at this point? Open for discussion and other proposals.

Example (really rough, please improve)

flowchart TD
    subgraph "repository_commits"
        rails_repo["Repository Rails"]
        db_repo["Repository DB"]
        cache_repo["Redis"]
        fe_repo["Frontend (HAML)"]
    end


    subgraph "gitaly"
    end
    
    gitaly --> rails_repo
    rails_repo --> db_repo
    rails_repo --> cache_repo
    cache_repo --> rails_repo
    rails_repo --> fe_repo