docs: Rework the development guide

docs: Rework the development guide

patchwork has undergone a number of changes recently that can make
deployment of a development environment much simpler than it was
previously. Unfortunately, these were not illustrated in the
documentation. Resolve this oversight by rewriting the developer guide
to reflect, for example, the usefulness of the 'parsearchive' script or
the fact that environment variables like 'PW_TEST_DB_USER' do not
always need to be exported.

This includes changes to the provided 'default_projects' fixture: the
existing projects weren't chosen for any particular reason and the
rewritten guide expects a 'patchwork' project to exist.

Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>