Add user guide page on how to use allocation and distribution utilities

[JoerivanEngelen]

Since we finished this: #628, we now have the cell allocation and conductance distributing utilities in place. A nice user guide explaining this functionality to uses is still required.

• Add/create some example data with planar grids containing river/ditch information
• Explain how to allocate cells over layers
• Explain how to distribute conductances over layers
• Plot cross-section to show results, show effects of different methods