This commit makes use of the virtual env in the `docs` target.
This way, all users have to do to build the documentation is run
`make docs`.

Change-Id: I5c4855d9d0a3fbbfeea53c568f3d9dc6de10ddd5
Tuleap: #636

This new documentation can be built using `make docs`. It should be
built automatically via a hook on github and published on
https://ring.readthedocs.org.

This allows for much better formatting and is more pleasant to browse
for new contributors. It is possible to build the docs using the
Makefile at the root of the repository.

Change-Id: I664d4662b76c525f2a67eac6266bada3e94aa835
Tuleap: #595