docs: fix of sphinx warnings.
docs: fix of sphinx warnings.

* Fixing all the warnings given by sphinx during compilation.

* Fixing the PEP8, PyFlakes and PEP257 warnings of the touched files.

* Adding some intersphinx.

* Reading the theme from docs/requirements.txt.

Signed-off-by: Yoan Blanc <7e8c88beed64bd6a341c3085bc82ac100e832f6e@cern.ch>

tiborsimko/invenio-base,inveniosoftware/invenio-base,tiborsimko/invenio-base,inveniosoftware/invenio-base