Add an accessibility statement

[ericwbailey]

This PR addresses https://github.com/ericwbailey/a11y-webring.club/issues/71. It:

• Adds an accessibility statement
• Links to it from the public site, and
• Adds a public-facing accessibility statement as a requirement for participation.

Additionally, it removes a stray file from a git merging issue.

Process-wise, I will check new applicants for the presence of a statement, and will reach out to current members if I cannot locate one.