Update documentation for canopy and crown

[davidorme]

Ongoing work on the canopy model (#315) is nearly ready to add as a PR. The new functionality on that issue will then need an update on the documentation for the crown and canopy modules. That documentation is currently in review (#313), so this is all stacking up a bit, but I wanted to keep a big pile of doc changes from bulking out the review of the code for #315.

The docs will need to be updated to:

• Help users understand the connection between crown radius and projected area better.
• Add a simple helper function to provide some more easily plottable structures for crowns.
• Extend the canopy description to talk about the light extinction and community crown area more generally before introducing the PPA model.

[davidorme]

This was closed by #328