neo_doxygen: Add a section about brief descriptions in the `README`.

Signed-off-by: Jean-Christophe Beaupré <jcbrinfo@users.noreply.github.com>
html: Add missing documentation.

Signed-off-by: Jean-Christophe Beaupré <jcbrinfo@users.noreply.github.com>
neo_doxygen: Add an API to process description markup.

Signed-off-by: Jean-Christophe Beaupré <jcbrinfo@users.noreply.github.com>