Update documentation

[corymharper]

Particularly on the accessibility page there are multiple dead links that have been moved in the authoring guidelines, many of which to either here or here. We should check each link on the page, and then for dead links find an appropriate replacement, if its a given we still conform to the information on that page. Also, as of #313 "The only deviation is that the first menu item is only focused when the menu is revealed via the keyboard (see why)" is now inaccurate and we should remove that bullet entirely.