Add user documentation infrastructure

The documentation is written using org-mode thus it requires Emacs to
generate html and pdf exports. To generate the pdf document, LaTeX is
also required.

The following targets have been added to CMake:
- userdoc-html: to build html doc
- userdoc-pdf: to build pdf doc
- userdoc: which builds both
- doc: which depends on userdoc and future develdoc