ship sphinx documentation in debian package

in #4 (closed), we undertook the task of merging all the documentation in the sphinx docs. this is great and very useful, but will be even more useful if documentation is also available in a -doc package locally, on request.

work has started for this in https://0xacab.org/monkeysphere/monkeysign/tree/dev/sphinx-build

then the help menu can point to the local documentation if available (see #44 (closed)).