Add syntax highlighting to docs via highlightjs

I've purposely modified "doc.tx" in Publisher so these files aren't needlessly loaded and executed on the rest of the Comm-Plat. Let me know if you agree with this approach.

I've tested this in duckpan publisher and it works as expected, adding these lines to the `<head>` only for the docs pages.

**Update:** I just realized the section I modified will only affect the preview in DuckPAN because it's wrapped in `<: if !$raw_output { :>` -- I'm not sure how else to make this apply only to the docs pages, other then putting in the `<body>` which is kinda ugly.

Any suggestions?

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An associated PR to the Community-Platform has been made which adds the required assets: https://github.com/duckduckgo/community-platform/pull/330

//cc @sdougbrown @russellholt @jbarrett