Add user documentation of solve interface

First stab with examples for solving and assembling linear and
non-linear systems along with a little bit on debugging convergence
failures.
Link solving docs into documentation page
Link to solving docs from Helmholtz demo
Add sections on modifying convergence tolerances
Silence sphinx warning
Linkify more things and expand schur complement section
Simplify introduction a little
Add section on default solver parameters
Typo fixes
More typo fixes
Merge pull request #129 from firedrakeproject/solver-docs