docs: Rebuild docs from scratch, as required for proper only:: handling.

Docs are now by default rebuilt from scratch, as required to build
conditionalized (i.e. using only:: directive) docs across different
output types. We have pretty small docset, so that's still rather fast.
However, if that's a concern, incremental rebuilds can be used by
passing "FORCE=" (nothing after =) as a make parameter. This will work
when using the same output type (e.g. only "html").