Integrate new Broxygen functionality into Sphinx.

Add a "broxygen" domain Sphinx extension w/ directives to allow
on-the-fly documentation to be generated w/ Bro and included in files.

This means all autogenerated reST docs are now done by Bro.  The odd
CMake/Python glue scipts which used to generate some portions are now
gone.  Bro and the Sphinx extension handle checking for outdated docs
themselves.

Parallel builds of `make doc` target should now work (mostly because
I don't think there's any tasks that can be done in parallel anymore).

Overall, this seems to simplify things and make the Broxygen-generated
portions of the documentation visible/traceable from the main Sphinx
source tree.  The one odd thing still is that per-script documentation
is rsync'd in to a shadow copy of the Sphinx source tree within the
build dir.  This is less elegant than using the new broxygen extension
to make per-script docs, but rsync is faster and simpler.  Simpler as in
less code because it seems like, in the best case, I'd need to write a
custom Sphinx Builder to be able to get that to even work.

doc/ext/bro.py

@@ -191,6 +191,10 @@ class BroNotices(Index):
 
     def generate(self, docnames=None):
         content = {}
+
+        if 'notices' not in self.domain.env.domaindata['bro']:
+            return content, False
+        
         for n in self.domain.env.domaindata['bro']['notices']:
             modname = n[0].split("::")[0]
             entries = content.setdefault(modname, [])