Add docs for installing dependencies and build on macOS

Updates COMPILING.md with macOS setup instructions.

Deps from Homebrew are mostly straightforward, but there are
a couple oddities due to interaction between system packages
and the Homebrew packages.

Fixes GNOME/librsvg#209
3 jobs for macos-docs in 1 minute and 26 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Test
passed #9795
fedora:test

00:01:26

 
  Lint
manual #9797
allowed to fail manual
clippy
manual #9796
allowed to fail manual
rustfmt