Link build and user docs to each other.

Point the build instructions to readthedocs.org and
the user guide to the repo's build instructions.
Run doxygen on readthedocs.org.

This should be sufficient for now. Eventually, it would
be better to make the reference documentation from a real
build and download the tag files and XML files to readthedocs
as required.
Merge topic 'readthedocs'

bf4fdfb Run doxygen on readthedocs.org.
00647f1 Link build and user docs to each other.
Err. **Really** only run doxygen on readthedocs.
Merge topic 'readthedocs'

242f130 Err. **Really** only run doxygen on readthedocs.