langref: rework the theming and layout

 * Instead of the only color scheme being dark, the language reference
   now has a light theme by default, and respects the user's light/dark
   preference via prefers-color-scheme media query. Most browsers don't
   support this yet, so we just have to wait patiently for the future to
   arrive. closes #2172.

 * Instead of a side bar index, the index is inline with the rest of the
   content. This is simpler and more friendly to all user agents, and means
   we don't need the media query for mobile devices. It also makes
   back-references work, so now headers link to the table of contents
   and the table of contents links to headers.

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