Add developer's guide to online docs

[1313e]

Make a new section in the online docs for developers, where the inner workings and structures of PRISM are explained, including:

• List of lists data structure;
• Emulator systems & data points;
• MPI rank assignments;
• Writing user/hidden/private methods;
• User exception handling;
• Coding style;
• Documentation style.

This will help current users to understand how PRISM works, and future contributors with how to make extensions.