features/modules: Expand modules section

I'd like some feedback on this sketch of modules patterns. I basically covered the simplest and most common design, but I'm sure there's some gaps in the structure here and there's definitely more material available to touch on.

I also think this should mention `mod foo { /* ... */ }`-style code within a single file. I'm not sure what constitutes a good use case for "inline modules" besides tests, and whether that should be discouraged altogether in favor of "real" modules.

All of these are listed **[OPEN]**, this is really tentative and so on, but any feedback would be super helpful.