Merge #11729: [docs] links to code style guides

fe27a7236 [docs] links to code style guides (Sjors Provoost)

Pull request description:

  I always have difficulty finding these, probably because they're not actually called "code style guideline".

Tree-SHA512: a2716c5e7e6778ce71b4451091e3899c0f9e04d07e95d1a0bdb07eb0319c083e3d9aa94bc2e33fc8546f82a1441e73c37218fdc7f228ee8273c7a87466958973

CONTRIBUTING.md

@@ -186,7 +186,7 @@ In general, all pull requests must:
     the project (for example refactoring for modularisation);
   - Be well peer reviewed;
   - Have unit tests and functional tests where appropriate;
-  - Follow code style guidelines;
+  - Follow code style guidelines ([C++](doc/developer-notes.md), [functional tests](test/functional/README.md));
   - Not break the existing test suite;
   - Where bugs are fixed, where possible, there should be unit tests
     demonstrating the bug and also proving the fix. This helps prevent regression.