Document function registry, material properties, boundary condition enforcement

[jeff-cohere]

I created a couple of stub pages in the Wiki where we can explain topics of interest that pertain to recent developments in TDycore. This issue can be closed when those pages are complete.

The Wiki remains our "short-term" documentation holding pen, I think, unless we prefer it to a more elaborate user guide underpinned by the Sphinx stuff. On the one hand, it's really easy to modify. On the other, it's very limited in terms of markup.

[jedbrown]

At the conceptual level, I prefer keeping the docs in Sphinx because it's better for cross-referencing and more likely to be updated when something changes. A couple of my projects are/have moved to MyST (a markdown dialect with feature parity+ relative to ReST), and that reduces friction.