initial support for multi-documention fetching

this is where doc/ contains multiple dirs rather than a flat structure with
one index.docbook.

there are a number of restrictions on this
a) it's either or, you can't have both
b) multi-dir doc directories *MUST* use optional_add_subdir, this in
   particular is necessary because the CMakeLists will be replicated into
   translations as to allow the use of branching based on available cmake
   packages for example
c) a doc folder that is not in the source will also not be included from
   the translations (kinda obvious)
d) all subdirectories are auto-supplied with CMakeLists.txt