Code Quality: Document entire repository

[Lamparter]

Similar to #15652, I've taken the time to document the entire repository. However, unlike the previous PR, this is far less descriptive codebase documentation and so is far less likely to become outdated.

To be honest, I wonder whether this might be more viable:

I guess we should xml comments and generate api docs from it (I know there's a tool for it).

That might be a good idea, but it's down to @yaira2 to express his plans for documentation.