Update CSS for DuckDuckHack Docs code snippets

- Reduce font-size to 12px from 14 for narrower code blocks
- Switch to better, cross-platform font-stack
- changes only affect docs pages, based on duckduckhack-docs class on
  <body>

This coincides with the changes to Community-Platform which adds syntax highlighting to the DuckDuckHack docs: https://github.com/duckduckgo/community-platform/pull/330

//cc @russellholt @sdougbrown