add "edit on github" to doc sidebar

based on the recipe found here:
   https://gist.github.com/mgedmin/6052926#file-edit_on_github-pyb

Note that it does not yet work correctly, but it looks like we
can tweak it to make it do what we want. I am just committing so
that it is clear what we have taken from the github gist and what
we have modified ourselves.
adjust github URLs

see also: https://github.com/rsyslog/rsyslog-doc/issues/70
remove "show on github" link as github cannot render correctly

the sphinx RST extensions generate error messages on github's
RST processor and so things looked extremly ugly and errornous.
Merge branch 'v5-stable' into v7-stable
Merge branch 'v7-stable' into v8-stable