distribute the glossary.rst file as package data

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

This should allow to stop requiring the swh packages tox files to install swh.docs in editable mode which is better and less prone to user specific setup issues.