[pkg] Tidy substvars usage in control file
The documentation package was missing a Built-Using field to record
the provenance of the templates.
Empty packages or meta packages do not require Depends on a specific
Python version, so we can remove the relevant substvars.
Web side development
====================
In this chapter, we will describe the core APIs for web development in
the *CubicWeb* framework.
.. toctree::
:maxdepth: 2
publisher
controllers
request
searchbar
views/index
rtags
ajax
js
css
edition/index
facets
internationalization
property
httpcaching
resource