oak_abi crate missing from generated docs

[daviddrysdale]

One slightly unexpected side-effect of #1034 – the online docs no longer include the oak_abi crate. This is arguably the most important crate to include in the documentation, so it would be good to get it reinstated.

This could possibly be combined with #765.

(Also, the generated docs still include the now-deleted oak_glue crate, which would probably just needs the dangling line in Cargo.toml to be removed – this change is included in #1036) [Edit: I thought I'd deleted oak_glue but I hadn't; now coming in #1050)

[rbehjati]

This is fixed now. oak_abi docs are available at https://project-oak.github.io/oak/oak_abi/.