Minimal docs.

[eddyb]

Before we release 0.1.0 on crates.io, we should make sure that everything is at least minimally documented.
That is, going to https://docs.rs/spirt/0.1.0 shouldn't look like a ghost town once we publish.

https://embarkstudios.github.io/spirt/spirt/ hosts CI-generated cargo doc output, so you can see what it would look like if we were to publish right away.

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Also, due to the use of macros for interned/entity types, some doc comments would go in unfortunate places, e.g. ControlRegion is only documented on ControlRegionDef, as the former is macro-declared in the context module.
It would be great if ControlRegion linked to ControlRegionDef or, even better, somehow held the docs itself.

It might be possible to attach docs to a reexport (instead of the original definition), in which case we could have e.g. pub use crate::context::ControlRegion; just above pub struct ControlRegionDef, and document the former.

[eddyb]

It might be possible to attach docs to a reexport (instead of the original definition)

Turns out it is possible - though only recently (thanks @jyn514 for the tip!):

• Documentation on re-exports is ignored if the item is defined in the same crate rust-lang/rust#84619

[eddyb]

While working on #16, learned that deny(missing_docs) is overkill (it mandates docs everywhere, including on fields, methods, associated types, etc.) - and IMO that's not what we should be focusing on for 0.1.0.