Deprecate Maruku in docs

As we use kramdown as Jekyll's default markdown engine now (#1988), it might be a good idea to update related documentation.

Questions:

- [kramdown](http://kramdown.gettalong.org/) states it's called `kramdown`, not `Kramdown`. (Personally I found this annoying though.) Do we need to reflect this in our documentation to use lower case?
- I'm not sure how we should deal with [LatexSupport section](http://jekyllrb.com/docs/extras/#latex-support) for kramdown. Should we simply remove this section (that link is 404 anyway)? Or figure out and document something like [LaTeX Converter](http://kramdown.gettalong.org/converter/latex.html)?
  I'm removing this section for now, but we can add it back in if someone actually has tested kramdown's LaTex support.
- Is there a reason we still have `redcarpet` as run-time dependency? This has been brought up in #2285, but @parkr didn't answer about `redcarpet`. (No actions will be taken in this PR though)