[RFC] Core feature documentation (Search, Snippets, Wagtail Tree, Editing API, etc.)

I'm new to writing formal documentation, so any criticism or requests to rewrite sections with certain goals in mind are welcome (hopefully with an example I can follow). I'm sure we'll need to work out some vocabulary, such as what shorthand to use when referring to "`Page`-derived models."

I've done my best to merge my work into the structure defined by the recent docs commits. If there was a different section structure in mind, it might be helpful to stub everything out on another branch.

I'm still working on the Editing API. I also stubbed out a few topics under Advanced Topics which I saw as useful but which wouldn't really fit anywhere else.