Include the doc dir in DIST_SUBDIRS instead of EXTRA_DIST

And explicitly specify what we want included in EXTRA_DIST of doc/Makefile.am,
otherwise make dist picks up whatever random junk you might have laying around
in that dir at the time you run it.

Tweak the top level delegation targets, because this means distclean will be
run in the doc dir by default now without our help.
diff --git a/doc/Makefile.am b/doc/Makefile.am
index 7bd13fa..e38e879 100644
--- a/doc/Makefile.am
+++ b/doc/Makefile.am
@@ -9,7 +9,7 @@
             $(top_srcdir)/doc/footer.html \
             $(top_srcdir)/doc/customdoxygen.css
 
-EXTRA_DIST = Doxyfile.in opus_logo.svg
+EXTRA_DIST = customdoxygen.css Doxyfile.in footer.html header.html opus_logo.svg
 
 
 if HAVE_DOXYGEN