Belos: Add a users guide
Created by: mhoemmen
Requested by @vbrunini. Current source of Belos documentation is Doxygen: https://trilinos.org/docs/dev/packages/belos/doc/html/index.html . This is a good start, but it's missing the following features:
- "How do I create a solver factory?" (Our recent DII work obfuscated the class hierarchy a bit.)
- "How do I make the solver that I want?", i.e., "What is the mapping from solver string names to solvers?"
- "What is a set of parameters that all Belos solvers accept?"
- "Given a Belos solver, what parameters does it accept?" (This information exists only in the Doxygen documentation; its location may not be obvious to users.)
We could start with the existing Doxygen documentation, as it's the easiest to find and already exists.