[clang-tidy] Add more documentation about check development (NFC)

- Mention pp-trace
- CMake configuration
- Overriding registerPPCallbacks
- Overriding isLanguageVersionSupported
- Check development tips
  - Guide to useful documentation
  - Using the Transformer library
  - Developing your check incrementally
  - Creating private matchers
  - Unit testing helper code
  - Making your check robust
  - Documenting your check
- Describe the Inputs test folder

Differential Revision: https://reviews.llvm.org/D117939