TS-2803: Use documentation strings extracted from source files in projec...

...t documentation
I rebased and added doxygen and python-lxml to contrib/manifests/debian.pp, next to python-sphinx. None of the other manifests mentions Sphinx.
I updated doxygen.py to issue warnings if the Python lxml library isn't installed but so far I haven't added anything to checkvers.py. If the library is missing, warnings are issued and the extra strings are just omitted from the built documentation.
I guess I'll have one or two tweaks to commit after we get a look at the Read the Docs build log, but based on what I've read it will work.