template: fix sphinx Makefile and tox

The idea is to add the location where swh-docs will install the
Makefile.sphinx directly as search path for the makefile when
building the sphinx documentation in tox.

With the modification of docs/Makefile to replace:

  include swh-docs/Makefile.sphinx

by:

  include Makefile.sphinx

this allows to have all the doc building scenarios working relatively
easily, without the need for adding ../..[...] paths here and there.

It should work ok when building:

- the whole doc from the swh-docs repo using the sphinx tox env; in this
  scenario, all the swh packages are installed from pypi and the
  location where to find this Makefile.sphinx if provided to the make
  command in tox (-I).

- the whole doc from the swh-docs repo using the sphinx-dev tox env;
  here, all the swh package source repositories are expected to be in the
  parent directory (swh-environment), and the Makefile.sphinx (included
  in all swh-xxx/docs/Makefile) will be found thanks to the -I argument
  of the make commande in the tox file.

- the doc for a specific swh package using tox; the tox file also set
  the -I option to the correct location where to find this
  Makefile.sphinx file.

- the doc for a specific swh package directly using the make command from
  a venv; here, either the user needs to put the -I option to the make
  command, or simply add a symlink to the swh-docs/Makefile.sphinx in
  swh-xxx/docs; this should be done automatically by
  swh-encvironment/bin/update script.

copier-template/docs/Makefile

-include swh-docs/Makefile.sphinx
+include Makefile.sphinx

copier-template/tox.ini.jinja

@@ -61,4 +61,4 @@ setenv =
   # turn warnings into errors
   SPHINXOPTS = -W
 commands =
-  make -I {env_dir}/share/ -C docs
+  make -I {env_dir}/share/swh-docs -C docs