Render octicons in code examples

[shawnbot]

This has been a really hard thing to figure out with the mess of MDX@0 (see: the migration guide) + our custom loader, Next.js (ugh), and various hacks that we made for react-live along the way. I think the most painless way to do this until we're using Gatsby and the new theme is to add a transform step at the very beginning of the MDX pipeline that replaces ERB <%= octicon %> helper text with an inlined <svg> element, then updates the fenced code block to have html as the language instead of erb (assuming there are no other ERB template tags remaining after the replacement).