gpio_common was built, but wasn't picked up by any documentation.
Use the peripheral_apis style from stm32, but try out the #pragma once
style. Downside is you don't get warnings if you include an sub layer
.h file. Upsides are
* no ifdef/endif blocks
* no /**@cond*/ /**@endcond*/ blocks
* that's enough win!
You can't have two mainpage items, and the second was just being
ignored. This restores them, which makes the left side list longer,
which we may or may not like, but it's at least how it was documented to
be.
