docs: fix demo links to playground

[Conduitry]

The redirect from a root REPL hash URL to the untitled playground only works on page load - so these links to /#H4... that happens via client-side routing will show you the homepage until you refresh. Maybe we could adjust how that redirection works, but in the meantime we should fix these links.

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[dummdidumm]

this is better anyway since it saves a redirect