CI doesn't build examples in documentation

[hannobraun]

All examples in documentation should be built by scripts/build.sh, but that currently does cargo build only, not cargo test.

cargo test needs the test crate, which isn't available on Cortex-M. Which means we need to compile the tests on the host machine, which is fine, since we don't need to run them. The problem is, the target is set in .cargo/config, and there's no way (as far as I can tell) to say "ignore that, I want the target to be the host".

This leaves us with several options, none of which are ideal:

• Not set the target in .cargo/config. This means we'll have to pass --target=thumbv6m-none-eabi to everything. This would suck, as I'm doing a lot of cargo run --example during development.
• Override the target by passing --target to cargo test. We could just pass x86_64-unknown-linux-gnu, which would be good enough for me and for CI, but not for the general case.
• Auto-detect target in build.rs and pass that via --target to cargo test. Maybe we can write the target triple into a file in target/, and read that in scripts/build.sh.
• Write a Rust program that prints the current target (using build.rs, as in the linked Stack Overflow answer), cargo install that in scripts/build.sh. Maybe something like that even exists?
• Implement Different default targets for cargo build and cargo test? rust-lang/cargo#6784 and use that new capability once it's released.

[hannobraun]

Forgot one additional option:

• Run rustdoc --test src/lib.rs directly. That doesn't require the test crate (I think), but we need to pass it all the info that Cargo would otherwise take care of.

[hannobraun]

I noticed some typos and wrong use of words in the title and original issue description. I updated both to make them more clear.