Make Wiki Markup editor's Full notation guide keyboard navigable

Editors who are blind could use assistance with the page editor. The full notation guide in concert with the Wiki Markup editor would serve them well, I think, except the full notation guide is not keyboard navigable. Please fix.

Secondarily, if a person who is blind attempts to use the Rich Text editor, they have to cycle through all the ribbon options as they are trying to get to the edit box, which was ~20 presses of the tab key in 2.10.3.

This should be considered a work stoppage for new wiki editors who are blind or sighted editors who can't use a mouse.