Add a proper comment to the github_url tag
Add url_segment Jekyll tag for treating page.url like an array

Yet again, we get bitten hard by both the paucity of data Jekyll makes
available to page templates and the paucity of logic allowed by Liquid.
I think we're fast approaching a point where we should be considering
other tools.

Anyway, if page.url is /references/3.0.2/type.html, this one lets us say
{% url_segment 1 %} when we want the 3.0.2 part.
Kill the one-trick reference_version tag in favor of the more flexible url_segment tag

This was being used to get PE versions and reference versions.
Exclude edit links from /references URLs

These documents properly live in the Puppet source code, so we don't
want them being edited in situ by helpful readers.