documentation: Fix markdown links.

All the github documentation was broken since the links ending with "html"
should have ended with "markdown". I tried my best not to hit any external links
using the following command from within ./documentation:

git checkout HEAD ./ && sed -ri -e '/http:\/\//!  s|([0-9_a-zA-Z]+)(.html)|\1\.markdown|' `find -iname "*markdown"`

If any links are still broken the fault lies in that janky little snippet.

Signed-off-by: Wayne Warren <waynr@sdf.org>