Docs needed: Hillslopes

[samsrabin]

What sort(s) of documentation issue is this?

• Something is missing.
• Something is (or might be) incorrect or outdated.
• Something is confusing.
• Something is broken.

What part(s) of the documentation does this concern?

• Technical Note (science and design of the model)
• User's Guide (using the model and related tools)
• Somewhere else (e.g., README file, tool help text, or code comment): Please specify
• I don't know

Additional context (optional)

#2425 will add instructions for creating custom hillslope datasets, but User's Guide instructions should also be added describing how to actually run with hillslopes (default or custom).