Executables now get built and deleted. Install doesn't do anything with
the executable, and --clean only deletes the executable itself (not any
support files or libraries that might also have been generated in the
exec directory)
This is an initial copy and refinement of the existing documentation for packaging.
This commit also starts the Idris2 reference documentation, in which we can start documenting the language proper. Maybe we should include the contents of `Notes` here, but that is a question for later.