Skip Links Example 404's

[darylldoyle]

Describe the bug
The best practices reference the Underscores theme when talking about skip links. Unfortunately, it seems that since Underscores have dropped support for IE11 they've also removed the skip-link-focus-fix.js file that is referenced (Automattic/_s#1424).

I'd suggest that either the link is updated to point to the last commit before its removal (available here), that it's moved to somewhere 10up has control of (Github Gist, inline in the skip link section etc.) or that another good example is found and linked to.

Steps to Reproduce

1. Go to https://10up.github.io/Engineering-Best-Practices/markup/#accessibility
2. Scroll down to "Skip Links" section
3. Click on the link within the "The popular Underscores starter theme comes bundled with a good option that can be used as a starting point." content.
4. See 404