[IMP] doc: PYTHONPATH configuration

Previously we'd `append` the extensions and odoo parent directories to
sys.path, however this means if an `odoo` is already installed in a
sys.path (e.g. global installation, or setup.py develop in a venv,
...) *that* will be used as the source of Python code (for autodoc)
instead of the Python source being synchronised with the rst, which
can lead to very odd behaviour.

Prepend the doc's stuff to the PYTHONPATH instead, to ensure they get
priority and the code matching the rst gets loaded.