Add contributor/developer documentation

[HighCommander4]

It would be nice to have some documentation geared towards clangd contributors. I know we have some info at https://github.com/llvm/llvm-project/tree/main/clang-tools-extra/clangd, but I don't think that's as discoverable as a page on the website would be.

Some topics to cover beyond what's on the page linked above:

• More details on clangd tests
  • The two types we have (lit tests and unit tests), and which one to use when
  • For lit tests, link to LLVM docs about lit and FileCheck
  • For unit tests:
    • How to run a single unit test with --gtest_filter
    • Some of the common utilities used in our tests, e.g. the Annotations class and its features
• Debugging clangd
  • Recommend writing a unit test and debugging that
    • For Linux users, maybe suggest rr
  • The next best thing after debugging a unit test is debugging clangd --check
  • If you must debug a clangd instance connected to a client, techniques to do that:
    • Find the process id of clangd process started by the client, attach a debugger to it
    • Alternative for rr users: point your editor to a script that wraps clangd with rr record clangd