Integrate message_doc

[glatterf42]

As discussed in the weekly meeting today, we want to integrate the documentation of the GLOBIOM model that is currently living under message_doc (docs, rtd, github) in the documentation of message-ix-models. But where/how exactly should that happen? We should

• [ ] leave behind redirects for people who saved bookmarks to the current docs
• [ ] do this before July 24 (ideally, first brown out), but definitely before October (full deprecation from RTD side of building docs w/o v2 config files)

[khaeru]

Here we can use a much-simplified version of the workflow in #89 to create a PR into message-ix-models.

Some specific steps I can imagine:

• The contents of the other repository can be moved into a new sub-directory of doc/ in this one.
• We discard the Sphinx configuration from the other repo, adding any bits necessary for this repo.
• We also integrate the index page, bibliography, directory of static images, etc. of the other repo within the corresponding locations in this one.
• Someone should do a brief skim/copy-edit of the text to ensure it is still coherent given its new location.

Unrelated to that PR:

• The existing message_doc builds can remain online in their current location until the PR is merged.