DO NOT MERGE YET move magic from documentation to papyrus

I've tested this: it allows to move conf.py from documentation to papyrus. It works for what I can quickly assess.

It works for the following:
- make html, debug-eng and the others that use ALLSPHINXOPTS
- make latex (well, it doesn't break worse than it already does)
- make gettext (which, for some reason, has it's own way of calling. Why? And what is it supposed to do anyway?)

however, I have *no* idea how to safely test against make transifex without ruining our transifex repository. And the transifex-options in the Makefile have their own -c options, which look like they are deliberately over-complicated. @giacomos can you have a look? And @svx can you think of things I'm missing?

Moving the magic to papyrus would greatly improve the esthetics of our setup...