Update outdated "Extras" page and remove duplicate documentation
Fix URL

Copy & paste error introduced a `.dev` local domain; change
to a relative path instead
Remove resolved RedCloth issue from troubleshooting
Added kramdown-with-pygments plugin

This plugin enables Pygments syntax highlighting for Kramdown-parsed fenced code blocks
Update the history I guess.
Merge pull request #2623 from mvdbos/master
Update history to reflect merge of #2623 [ci skip]
Merge pull request #2622 from alfredxing/fix-markdown-docs
Update history to reflect merge of #2622 [ci skip]