Create a "Code quality checks" document

[NicMcPhee]

There used to be a "Code quality checks" document in Lab 4 that explained what BetterCodeHub (BCH) and LooksGoodToMe (LGTM) were doing. We're not using either of those anymore, so that whole document got removed last year.

It would probably make sense to create a replacement that talks about what we're using now (CodeQL and DeepSource), and then copy that forward into the remaining labs.