Provide a declaration of accessibility

There should be a declaration of accessibility available in the XWiki footer.

IMO this should redirect to a page on the xwiki.org documentation.
This should also be toggable, so that admins can remove it easily if they don't think it fits their wiki.

This issue was first reported by Zendis in their accessibility audit.

Note that this isn't technically a WCAG standard, but it's a part of several national standards about accessibility (at least BITV and french legal obligations). Tagged as wcag because it's the only tag for accessibility