Improve LLVM backend documentation

[Baltoli]

Currently, the LLVM backend code is only really documented in-source; this means that getting up to speed with it can be a bit opaque. We have nice documentation for the pattern-matching algorithm part of the compiler, but it would be good to write up some high-level documentation for how each step of the backend works. This could potentially take place alongside some code cleanup.

Work done:

• Sketch out documentation structure #650