[docs] Add "GNU binutils Replacements" section to command guide
This splits out a section in the command guide for llvm tools that can be used as replacements for GNU tools. For pages that didn't exist, I added stub pages that can be individually filled in by followup patches.
Tested by running ninja docs-llvm-html and inspecting locally.
Reviewers: jhenderson, MaskRay, grimar, alexshap
Reviewed By: jhenderson, MaskRay, grimar
Subscribers: smeenai, arphaman, llvm-commits
Differential Revision: https://reviews.llvm.org/D63014