Simplify building of documentation

The main issue in building the docs comes from using sphinx.ext.autodoc, which need to import the documented modules to extract the docstrings.

Previously one had to create a hackish venv manually based on LbEnv and call Sphinx in a special way. In this MR the opposite approach is taken -- Sphinx is installed in a dedicated directory on top of Moore's runtime environment. In addition, the preparation of the environment is integrated with the Makefile so that one can just do

Moore/run make -C Moore/doc html