docs: Rewrite documentation

docs: Rewrite documentation

The INSTALL and HACKING documents are an important guide for new
patchwork users and developers and should be as informative as
possible. A number of changes were needed to these documents owing
to the out-of-date or incomplete information they contained. These
changes include:

* Removing references to the dead mod_python/flup projects
* Adding references to Gunicorn+nginx, which a credible modern
  alternative to Apache+mod_wsgi
* Providing explanatory links to concepts/tools like ident-based
  authentication and tox
* Referencing the newer tools available to developers, like tox
  and the 'requirements.txt' files
* Integration with mkdocs, with eye on eventual publishing of
  documentation to ReadTheDocs or equivalent.

These changes result in a significant rewrite which should hopefully
lower the barrier to entry for people wishing to use or develop
patchwork.

Acked-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>