René Paris authored Sep 28, 2021 and Reinhard Biegel committed Mar 31, 2022

This commits adds the directive `toctree-api` to sphinx, enhancing the
regular toctree by a conditional `:api-doc:` entry. If `api_doc_build`
is activated (= `cmake --DWITH_API_DOC=ON`), the api-doc extension
(a) inits and configures the sphinx extensions `breathe` and `exhale` to
generate the API doc and (b) inserts the api-doc into the toctree, where
the key `:api-doc:` is found.

Signed-off-by: Rene Paris <rene.paris@in-tech.com>