Infrastructure for documentation translation

Provide an infrastructure similar to what the PHP manual did with DocBook:

Have different language versions of the documentation in parallel document tree subdirectories with one of them ('en') being the 'master' language.

When generating the documentation for a specific language scan the translation tree for that language first and collect all document files in there, then scan the 'master' language tree for document files present there but missing from the translation tree. This way a localized manual is alway 'complete' in that for untranslated files the 'master' language version is pulled in instead (assuming that a mixed language manual is still better than an incomplete one).

This actually requires Sphinx modifications so that it can be given a source search path instead of just a single source directory, e.g.:

  sphinx-build ./docs/de:./docs/en ./docs/html/de