[docs] Rewrite the docs for diff

The previous docs were outdated, and recent patches specified more
cornercase that were abscent. I decided to rewrite and exand on the diff
docs.

As I wrote more and more, it made sense to create a brand new file. The diff docs for specifically the GUI (https://github.com/Szelethus/codechecker/blob/diff_docs_rewrite/web/server/vue-cli/src/assets/userguide/userguide.md#compare-runs) seem to be a mess, but I'm not sure there is a need for rewriting, and if there is, I'm pretty sure it can wait a little :)

Check out the final docs here: https://github.com/Szelethus/codechecker/blob/diff_docs_rewrite/docs/web/diff.md