move api reference navigation to sidebar

[thisisjofrank]

The API reference sidebar feels ugly and clunky, I've moved it over to the left hand sidebar and grouped the three "sections" under one menu

I have also moved each of the 'api landing pages' that are currently in runtime/reference/ into the api reference section, so, for example
https://docs.deno.com/runtime/reference/deno_namespace_apis/ becomes https://docs.deno.com/api/deno/

I've also made the accordions collapse when a different accordion is expanded and now the sidebar will scroll to the correct position when a page is loaded to show the 'active' link in the browser viewport.

Old:

New:

[philhawksworth]

I really like this as a visual update. It feels like it adopts and stenghtens the conventions about showing hierarchy around the docs site

It also lends itself well to having an intro explanatory page for each section which is present in the sidebar nav too.