docs: improve site accessibility

[jaredcwhite]

Given the Bridgetown website is pretty standard HTML markup for the most part, I don't think we have a ton to do to improve accessibility, but there's definitely some low-hanging fruit. We need a "Skip to main content" link for example (and it's extra annoying in that the first item in the DOM is the theme switcher, so landing there on every page navigation sucks).

There's also an issue in the search menu I documented here: bridgetownrb/bridgetown-quick-search#20