mk: Fully clean out old documentation

This purges doc/{std,extra} entirely during a `make clean` instead of just the
html files in some top level directories. This should help old documentation
from showing up on static.rust-lang.org
auto merge of #9544 : alexcrichton/rust/clean-docs, r=pnkfelix

This purges doc/{std,extra} entirely during a `make clean` instead of just the
html files in some top level directories. This should help old documentation
from showing up on static.rust-lang.org