[libc++] Update contributor documentation.

The document has the following updates:
- Rename 'feature test' to 'feature-test', the latter is the spelling
  used in the Standard.
- Add information how an ABI list can be downloaded from Buildkite.

Differential Revision: https://reviews.llvm.org/D99290

GitOrigin-RevId: 48fa06f70b07fc195a07bfb67ee42cfd307ca450
1 file changed