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").