Fewer deps for 'make webdocs' from top level

Previously it also built the MANPAGES target
requiring asciidoc and libxml, before starting
a 'make docs' in docsite.

Also change the #! line in
hacking/dump_playbook_attributes.py to not specify
python2... yet.

hacking/dump_playbook_attributes.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python2
+#!/usr/bin/env python
 
 import optparse
 from jinja2 import Environment, FileSystemLoader