RAL doc string fixes and doc test enablement

[teburd]

Currently CI has doc testing disabled for RAL because the doc strings contain things like stm32ral::gpio type use statements, as well as a number of stm32ral specific types, modules, and such.

It'd be really nice to fix this so we can having tested rust code in our doc strings for RAL, though not a super high priority as most of the people working on the HAL I believe implicitly understand the nature of things at this point.

When fixed we can re-enable cargo test in CI for doc strings.