Generated Rust docs include detritus from examples

[daviddrysdale]

Since #613 & #616, the generated docs for Rust crates at https://project-oak.github.io/oak include docs for every crate in the codebase, including pointless & empty pages for all of the examples.

The doc page would be much more helpful for Oak developers (both of and on Oak) if the example crates were dropped.

Ideally, it might also be nice to separate out:

• docs for developers on Oak (oak_abi, oak, oak_tests)
• docs for developers of Oak (oak_runtime, oak_glue)

(This is roughly the separation that older versions of the gh-pages branch had, between the sdk/ and server/ subdirectories, but it's not completely clear to be where the oak_abi crate should live with that division.)

[daviddrysdale]

I think this is done already (by #1042, improved further by #1122).