Technical documentation

This is technical part of the documentation, intended primarily for DSL developers (in contrast to DSL users). The docs here differentiate frontend and backend, give tips for your development setup and include notes on the translate tests system used to port FORTRAN code to NDSL.

We have many ideas on what to do in the future. Since the work is so fragmented, across repositories, we started a laundry list in the hope keep things together in one place.