Cranelift: modernize documentation

[cfallin]

We should update our documentation in cranelift/docs at least in the following ways:

• We should make a pass to ensure that outdated concepts related to the old backend framework, like encodings and recipes, are removed;
• We should fill out documentation about the new backend framework's architecture, beyond the doc-comments in the source (but likely borrowing from them);
• We should provide more "purpose-directed guides":
  • How to add a new backend (Cranelift: make authoring a new machine backend simpler, and document it #4126);
  • How to add a new instruction to CLIF;
  • How to add a new instruction to a machine backend;
  • How to add a new lowering from CLIF to a machine backend;
  • Strategies to debug codegen issues (minimization, diffing output, using "optimization fuel" (Cranelift: test the compiler with a "chaos mode" #4134)).