build docs on readthedocs

[aewallin]

use an automated on-line documentation system like readthedocs.

ideally this should build documentation for both the c++ and python APIs, e.g. based on doxygen for c++ and docstrings for python.

[aewallin]

now trying this:
https://breathe.readthedocs.io/en/latest/index.html

we first generate an XML file with doxygen.
breathe is then used to convert the XML to sphinx format.
documentation can then be included in sphinx RST-format documentation (how could this be automated?)

currently the readthedocs build fails because it doesn't import breathe:

ExtensionError: Could not import extension breathe (exception: No module named breathe)

Extension error:
Could not import extension breathe (exception: No module named breathe)

[vespakoen]

I think this can be closed too =) just need to update the docs now =P

[aewallin]

thanks! closing.