`An Alphabet of Accessibility Issues` is no longer reachable

[francisfuzz]

Summary

In https://primer.style/design/guides/accessibility/accessibility-at-github, there's a reference to An Alphabet of Accessibility Issues. However, the domain is no longer reachable.

We should update this reference so that it renders a version that end-users can consume, like:

https://web.archive.org/web/20230307004829/https://the-pastry-box-project.net/anne-gibson/2014-july-31