Distribute these Makefiles in share/swh-docs and move the
glossary.rst in share/swh-docs/docs/devel to keep the directory hierarchy
of the source repo.

Distribute this docs/devel/glossary.rst file as package data in
share/swh-docs since it is needed for locally building the doc
from swh packages.

Adapt the add_glossary_to_index sphinx hook to look for this file in
share/swh-docs in addition to the current relative path (which only
works if swh-docs in installed in editable with proper pip
config-settings and who knows what else). For local/editable installed
swh-docs, look a bit more than previously to attempt to catch the case
of editable_mode=strict pip config

There are plenty of other phabricator links in another places in the
doc, but this ones are the first encountered by a new comer.

Related to swh/devel/swh-docs#4732

Related to T2610

Related to T2105

Related to T2367

to prevent weird race condition when both requirements-swh.txt and
requirements-swh-dev.txt are added as requirements.

This is needed to make:

  tox -e sphinx-dev

work as expected, ie. with swh packages installed from sources rather than
from pypi.