feat(www): Add link to accessibility statement

[marcysutton]

Summary

PR #13190 adds a new page for an Accessibility Statement, but we decided to ship it without a link on the homepage until it gets some design attention. This issue is a reminder to do that, so people can actually find the accessibility statement!

My proposed change (now removed) put a centered link at the bottom of the homepage. It would also be a good place to move the .com link potentially, as it would create more space at the top of the page and we could spell out "Gatsbyjs.com" like discussed in #12619.

Ideally, the footer would exist on all of the .com site so that the links could be reached from any page. But the docs and plugins pages go full-height, and a plain text footer would have to be worked in somehow. @fk would love to get your input on this!