Documentation TODO: easy linking between docs and code

[thundergolfer]

At the moment if you're on https://thundergolfer.com/uni it's not simple to look at the code associated with each README (_index.md) file.

A user should be able to land on a page like https://thundergolfer.com/uni/docs/operating_systems/linux/strace_in_seventy_lines/ and quickly jump to https://github.com/thundergolfer/uni/tree/master/operating_systems/linux/strace_in_seventy_lines

Should I prepend a header to each readme.md file in update_doc.sh that contains a listing of the sibling files? Like how GIthub does it. Or should I just link to Github directly with 'See The Code'.