Add prerequisite sections to a few places

[knownasilya]

This bit me, I started updating and a ways into the update got the startTransition is not a function error. Almost on 18, but not yet, so a note on the docs site would have saved some frustration/time.

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[brookslybrand]

Thanks @knownasilya for adding this. I made a couple of changes:

• Move the callouts to <docs-layout> (see screen shot) instead of an additional h1
• removed installGlobals -- this was a Remix thing, so mentioning it on the React Router upgrade docs only has the potential to add confusion. It's discussed in the Remix future flags doc, which is step 1 of upgrading from Remix