Linear Solver Documentation Improvements Suggestions

[MelReyCG]

What needs to be documented?

When trying to setup a simulation from scratch, I found that linear solver settings were not that easy to choose.

• Would it be possible to have some presets for some standard scenarios ?
• The options of the solverType are not documented in either the linear solver documentation or the enum code. What are each one pro and cons ?
• All the preconditioner compatibilities are not documented in the linear solver documentation. What are each one pro and cons ?
• Some key settings in relation with the preconditioner and the solverType options are not documented.
• There are some preset that are available in tutorials, are they up to date with what is available in GEOS ?

[paveltomin]

CC @castelletto1 and @victorapm