api doc: add a working showdown (Markdown) example

I couldn't figure out an easy way to include a Spacebars operator like
{{#markdown}} in a Markdown code block, hence the <code>{&#123;#...
trickery. I think I can clean this up a little with the {{dstache}}
template helper, though. See next commit.
api doc: use {{dstache}} template helper

Less/cleaner source code.
Merge branch 'pr-2673-merge' into devel