add `setup/materialize-user-docs` and related `raco setup` flags

The new library provides a way to force a user-specific documentation
etry point into existence. Normally, that would happen when a package
with documentation is installed in user scope. After the entry point
exists, then it sticks around even if all user-scope packages are
removed. In some cases, it may be useful to force the entry point into
existence as if packages had been installed and removed.

(This might be useful for avoiding a quarantine on installed
documentation files on Mac OS X, or a trampoline might be better.)