Add docs for Mach-O lld

I wasn't able to find any docs for Mach-O in `lld/docs`, so here's an attempt at adding basic docs. One of my goals here is to make it easy for users who are unfamiliar with linkers to successfully use lld.

Reviewed By: #lld-macho, int3

Differential Revision: https://reviews.llvm.org/D132893