Update configuration docs for MD extensions

Per `.mkdown` extension addition in my older pull request.
Update Site history based on History.markdown
Merge pull request #2102 from ZDroid/mkdown
Update history to reflect merge of #2102 [ci skip]
Update plugins.md

Added my plugin to the list.
Merge pull request #2105 from callmeed/patch-1
Update history to reflect merge of #2105 [ci skip]