Restore heading anchor links in style guide

Previously we had an easy way to get a deep link to a particular heading. This is no longer visible, but is key to the recommended reviewer workflow with the style guide. The anchors are still present. This is mitigated by the availability of the right sidebar TOC.

Presumably an update to the docsy theme removed this functionality.

1. See if there is a docsy option to bring this back
2. Override a template to customize this back into our site.