docs: add a section on "Learning the codebase" to CONTRIBUTING.md

[agilgur5]

Summary

Add a sizeable "Learning the codebase" section to the contributing docs to help new contributors!

Details

• thought that as it's somewhat? fresh in my head (pending if you consider years of sporadic contributions), it would be really good to document for new contributors who don't know where to start

• other than being good practice, thought this could be useful for a number of reasons:

  1. helpful resources like the TSConfig Reference, the TS Compiler Wiki, and the Rollup Plugin docs, that I read through with some frequency
     • (and I contribute lots to the TSConfig Reference too, it's by far the most useful resource in the ecosystem, IMO)
  2. explaining directly in the docs how sparsely documented the TS Compiler API is
     • like it really makes things difficult and every time I'm looking to contribute a bigger bugfix I look at the TS Wiki and don't get a super helpful answer there
  3. this codebase is actually fairly simple (it's the main reason I became a contributor! as I've stated in a few issues) and truly not that hard to get started with
     • but there are some rabbit holes you can go down that can scare away some contributors, like the cache code or the logging nuances
       • so this gives a bit of a guide to not fall into some rabbit holes and start off with the less complex bits of code
  4. encourage more contributors to make some PRs like I first did!

Review Notes

Feel free to add your own recommendations here! And let me know what you think of my take and the resources I use; I'm assuming we use similar docs/references, but who knows

[ezolenko]

Heh, Language Service API documentation does leave a lot to be desired...