doc: Update requirements-doc.txt to use docleaf

Instead of breathe.

I'm not sure of the best version comparison to use. There is no
intention to break the API for docleaf but it is also still young
in some ways and there might be adjustments to the configuration values
that would need to be reflected in the version number as it is still
pre-1.0 that would only be a minor version change.

Signed-off-by: Michael Jones <m.pricejones@gmail.com>