Update Cranelift's documentation after the merger. (#1238)

Update the documentation for the merger, and also for various changes in
Cranelift. Remove some old obsolete documentation, and convert the remaining
Sphinx files to Markdown. Some of the remaining content is still out of
date, but this is a step forward.

cranelift/simplejit/Cargo.toml

@@ -4,7 +4,7 @@ version = "0.59.0"
 authors = ["The Cranelift Project Developers"]
 description = "A simple JIT library backed by Cranelift"
 repository = "https://github.com/bytecodealliance/cranelift"
-documentation = "https://cranelift.readthedocs.io/"
+documentation = "https://docs.rs/cranelift-simplejit"
 license = "Apache-2.0 WITH LLVM-exception"
 readme = "README.md"
 edition = "2018"