Documentation of the TH2M/H2 McWorther benchmark

The following discussion from !3798 (merged) should be addressed:

• @endJunction started a discussion: (+4 comments)

  The resulting saturation profile can be compared using a (semi-)analytical solution (http://mmg.fjfi.cvut.cz/~fucik/index.php?page=exact).

  How is the OGS solution compared to the semi-analytical solution?

A corresponding documentation page with plot against the semi-analytical solution should be set up discussing the differences. Plots of the solution and the error are to be added.