gridpp/gridpp-userguide-doc: Initial version (v1.0).

The GridPP Collaboration is a community of particle physicists and computer scientists based in the United Kingdom and at CERN.  It supports tens of thousands of CPU cores and petabytes of data storage across the UK which, amongst other things, played a crucial role in the discovery of the Higgs boson at CERN's Large Hadron Collider. The aim of the document - an offline version of the online GridPP UserGuide - that is built by this code is to help new users join this community and access these resources to make a difference to the world beyond the realm of particle physics.