restore "getting started" link

This change adjusts the meaning of the 'getting-started document
category, which was formerly omitted from the start page. Now,
it means a document that is listed at the beginning and at the same
level as other documents.

(Didn't just revert to the old approach to the "getting started"
entry, because it doesn't work right when "racket-index" is installed
without "racket-doc".)

Closes PR 14348