Finish port to sphinx

[Ivoz]

The sphinx branch is being used so that translations can go ahead.

• Update lost links (at least for the resources section)
• Create translation infrastructure
• Synchronise with latest gh-pages head
• Give a bit of OTS branding

[lehmannro]

If you need any assistance with the translation infrastructure feel free to
ping me and we can house it under the existing Transifex organization.

Re branding, maybe it's useful to customize one of the themes (and push
that to a separate repository) so that all OTS Sphinx projects can use it?
(Also the LaTeX builds could benefit from some nicer branding.)