[globalisel][docs] Rewrite the IRTranslator documentation

[globalisel][docs] Rewrite the IRTranslator documentation

Summary:
I haven't refreshed the Function Calls section as I don't feel I have
sufficient knowledge of that area. It would be appreciated if someone could
review that section.

Note: I'm aware that pygments doesn't support 'mir' as used in one of the
code-block directives. This currently emits a warning and I decided to
keep it to enable finding them later. Maybe we can teach pygments to
support it.

Depends on D69456

Reviewers: volkan, aditya_nandakumar

Subscribers: rovka, Petar.Avramovic, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69457