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 Some of the remaining content is still out of date, but this is a step forward.
This is an umbrella crate which contains no code of its own, but pulls in other cranelift library crates to provide a convenient one-line dependency, and a prelude, for common use cases.