Create a short ForTrilinos User's Guide

[aprokop]

The content could be:

• Installation
  • Take installation instructions from existing RTD
  • Talk about spack option (btw, this will have to be updated)
• Brief overview and the general gist
  • Try to steer users to the existing Trilinos documentation
  • Highlight the differences (like native Fortran arrays, and few functions with different semantics)
• Provide few examples
  • Could use existing power method example
• Talk a bit about performance