doc: use common naming for peripheral apis

Only applied to STM32 doc trees at present.

Instead of declaring a group for "STM32blah" in the doc-blah.h files,
and then trying to put all the common+specific peripheral code into
those groups, (which is what led to the stub doxygen holder empty .c
files)  Just use a standard name like "Peripheral APIS" and place
everything into that.

Demonstrated by converting ADC and USART peripherals, which is
definitely not complete, but it shows how to make things less magical,
and less prone to copy/paste errors.  Now, you can copy/paste and it
will do the right thing, because everyone uses the same group names.

This is also how to unify the mix of "STM32blah->Periphblah" and _also_
the dangling "periph_file" modules in doxygen, it merges them together
properly, as they're intended to be really.

lib/stm32/f0/adc.c

@@ -1,8 +1,5 @@
-/** @defgroup adc_file ADC
- *
- * @ingroup STM32F0xx
- *
- * @brief <b>libopencm3 STM32F0xx Analog to Digital Converters</b>
+/** @addtogroup adc_file ADC peripheral API
+ * @ingroup peripheral_apis
  *
  * based on F3 file
  *