Fix broken link; update other links related to accessibility review in the Guide

[This section of the guide](https://www.w3.org/guide/documentreview/#how-to-get-horizontal-review:~:text=w3c%2Dwai%2Dig-,links,-Framework%20for%20Accessibility) contains a broken link to the FAST checklist. It also links to the MAUR, but maybe it should link to the [list of all *AUR documents](https://www.w3.org/WAI/research/user-requirements/) instead?