docs: add basic info about solvers to users guide
docs: add basic info about solvers to users guide

lokik/sfepy,RexFuzzle/sfepy,BubuLK/sfepy,RexFuzzle/sfepy,RexFuzzle/sfepy,lokik/sfepy,vlukes/sfepy,lokik/sfepy,rc/sfepy,vlukes/sfepy,BubuLK/sfepy,lokik/sfepy,vlukes/sfepy,rc/sfepy,rc/sfepy,RexFuzzle/sfepy,sfepy/sfepy,BubuLK/sfepy,sfepy/sfepy,sfepy/sfepy