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 Some of the remaining content is still out of
date, but this is a step forward.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/wasm/src/environ/spec.rs

@@ -71,7 +71,7 @@ pub enum WasmError {
     /// Cranelift can compile very large and complicated functions, but the [implementation has
     /// limits][limits] that cause compilation to fail when they are exceeded.
     ///
-    /// [limits]: https://cranelift.readthedocs.io/en/latest/ir.html#implementation-limits
+    /// [limits]: https://github.com/bytecodealliance/wasmtime/blob/master/cranelift/docs/ir.md#implementation-limits
     #[error("Implementation limit exceeded")]
     ImplLimitExceeded,