doc: api: sort the API overview alphabetically

This makes it easier to scan. The sorting is done based on the text
that appears in the HTML, not the :ref: label name.

In the case of the DMIC API, change the title of the document to match
the "Audio Codec" document so we can keep those together.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>