documentation: added client docs structure

Signed-off-by: Cleber Rosa <crosa@redhat.com>
client/shared/distro.py: introduce distro detection API

This new API offers a way to precisely detect not only the vendor
OS, but also specifics such as the version and release of the
currently running distro.

Probes for new distributions are quite simple to create. That and
the basic usage of the API is properly documented.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
client/shared/software_manager.py: use distro detection API

Signed-off-by: Cleber Rosa <crosa@redhat.com>
client/shared/distro.py: add Probe based on Python's Standard Library

Ross Brattain pointed us to the platform.dist() function on Python's
Standard Library. We realized it's valuable to use it in addition
to our custom Probes.

So this introduces a Probe that relies on platform.dist(), that will
have its result used if no other (more specific) Probe exists.

Signed-off-by: Cleber Rosa <crosa@redhat.com>