Starting to cleanup the garbage collection documentation

Starting to cleanup the garbage collection documentation

In this change:
- Put the getting started section first
- Create a dedicated section to document the built in collector strategies
- Move discuss of ShadowStack into new section
- Add placeholders for erlang, ocaml, and statepoint-example collectors

There will be many more changes following.  I plan on full integrating the documentation for gc.statepoint and gc.root.  I want to make it much clearer on how to get started and what users should expect in terms of effort.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230359 91177308-0d34-0410-b5e6-96231b3b80d8