[doc] correct pip command to have needed dependancies to use the start command
I know we want to remove this command but in the meantime having this
documentation working seems like something good.
Additional services
===================
In this chapter, we introduce services crossing the *web -
repository - administration* organisation of the first parts of the
CubicWeb book. Those services can be either proper services (like the
undo functionality) or mere *topical cross-sections* across CubicWeb.
.. toctree::
:maxdepth: 2
undo