Deprecate absolute paths in src/multi-verso/MultiVerso/Path.lean

[jcreedcmu]

The approach to solving #555 means that we probably should not be generally generating paths that start with / if we can possibly avoid it. Replace the 7 or so uses of the link function in src/multi-verso/MultiVerso/Path.lean and delete it.

Be sure to update the docstring of the Path type (which currently has an absolute-path example) to reflect this change of contract.