Add syntax highlighting to DuckDuckHack docs

This adds the `highlight.pack.js` lib (a special build for the languages we need -- https://highlightjs.org/download/) and the `tomorrow-night-eighties.css` syntax theme to the comm-plat assets to allow for Syntax Highlighting in the docs. I wasn't entirely sure where to put these assets, please let me know if they belong somewhere else.

I've also updated `head.tx` and `scripts.tx` to conditionally add the required `<link>` and `<script>` tags to the page,  which enable the syntax highlighting.

I've made a PR to community-platform-static that adds a few lines of CSS to build upon these changes: https://github.com/duckduckgo/community-platform-static/pull/2

//cc @sdougbrown @russellholt @jbarrett @jdorweiler @chrismorast