unknown file warning can be noisy

[devoncarew]

The warning warning: dartdoc detected an unknown file in the doc tree: can be very noisy; should we reconsider having this warning? Should we have dartdoc delete doc/api/ before doc generation?

[jcollins-g]

I'm OK with deleting the doc tree before generation, but was not certain that wouldn't break some obscure dartdoc workflows so I put a warning out first to figure out who that might annoy. I suggest we leave it as a warning and see if anyone else notices/complains for a while before adding the removal.

[sigurdm]

Still seems relevant - but low priority