Cranelift: modernize documentation

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* We should make a pass to ensure that outdated concepts related to the old backend framework, like encodings and recipes, are removed;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 (#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" (#4134)).