doc: reference: Add a section for the C standard library

In order to document the error codes, introduce a new reference section
describing the C standard library options available in Zephyr, along
with the definitions of the error codes.

At the same time move (most of) the content from the existing user guide
to the new API reference section, removing the need for a separate user
guide for it.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>