Clean _build directory before building new docs

Currently non-deploy docs, built with the pretty_urls option disabled,
also get pushed to gh-pages. With this, they will be cleaned away before
the deploy docs are built and we should end up with a single set of HTML
pages on the gh-pages branch (as opposed to the current e.g.
base/file.html AND base/file/index.html).

doc/make.jl

@@ -153,7 +153,7 @@ end
 makedocs(
     build     = joinpath(pwd(), "_build/html/en"),
     modules   = [Base, Core, BuildSysImg, [Base.root_module(stdlib.stdlib) for stdlib in STDLIB_DOCS]...],
-    clean     = false,
+    clean     = true,
     doctest   = "doctest" in ARGS,
     linkcheck = "linkcheck" in ARGS,
     linkcheck_ignore = ["https://bugs.kde.org/show_bug.cgi?id=136779"], # fails to load from nanosoldier?