[DO NOT MERGE] #19898 - Document why/when of class-based views 

Beginning the pull request early for documentation updates for class-based views.

Currently:
* I've written out some thoughts that I think are important to express on the front topic page, but I need to do some work so weave this new content into the rest of the page better (i.e., make sure there's no overlap/repetition)
* Currently working on the usage-patterns page that will hopefully demonstrate the ideas stated in this new topic front-page material.