Add random ray theory & discussion to documentation

[jtramm]

Currently the documentation on the random ray solver is nearly non-existent. The only documentation is some terse discussion on the settings themselves, with no overview of what the solver, how it works, or how it should be used. It would be good to greatly expand the docs with a new section on the random ray solver.

[jtramm]

Most of the basic theory and documentation for random ray is going in with PR #2823.

[paulromano]

Closed with #2823