Update Accessibility statement following feedback from HO Access team

[edhamiltonHO]

• [x] Include link to HO Accessibility Standard
• [x] Add feedback and contact information section
• [x] Include in the non-accessible content that contributing to the site is not necessarily accessible, with mitigation that people can email segas@. Point to GitHub Accessibility conformance report, note that out of scope for our testing and assurance https://accessibility.github.com/conformance/github-com/
• [x] Check all parts of HO Accessibility template to ensure all relevant elements have been included
• [x] Add reference that accessibility checks are based on assumption that continues to be a static site