Part of #1139

Foster the "Edit me on github" link:

• Add the button additional at the bottom with some more explaining text, e.g. "This
  documentation is writting in markdown. If there is something missing or can be improved,
  edit this page on github and create a PR: [button]".
• Add a smaller button in the section heading line of each section
• Remove references to the sourceforge forum ("... If you think this can be improved...").