setup new manual in simple markdown format

this is still in flux, I need to
- implement translating paragraphs and list items and probably link texts
- automatic substitution of things like {key_help}, {author}, {menu_file_open}
- implement a way to mark pieces of md not to be translated, e.g. that
  {name:Wilbert Berendsen} is changed to {name} in the translatable string
  and that the value is inserted before display. (this is an example)

The end result will be that adding a help page is as simple as putting a
markdown file in the userguide/ directory, and that translatable strings
will be automatically extracted in a sensible way and added to the pot file
(just by creating a dumb .py file with all the messages).