Theory-specific documentation

[KevinDCarlson]

There aren't many instructions; probably we should at least have something like a sidebar reminder of what keyboard commands are available and, dependent on the theory, of what kinds of cells fit into what other kinds of cells.

[epatters]

Infrastructure for this feature has been added in #278 (thanks @hamidahoderinwale!). Now we need to actually document the theories!

[epatters]

With #307, there is now a reasonably complete example of a help page, namely for ologs. I'd say that the main thing that is missing are linked examples (see #296).