Clarify when the import flow and Farcaster manifest are still needed

[gizdusum]

This adds a small clarification to the mini app docs for developers who are unsure whether they should follow the standard web app path or the older manifest/import flow.

The main issue is that it is easy to read the docs as saying both of these things at once:

• standard web apps in the Base App no longer require a Farcaster manifest
• existing apps should still add /.well-known/farcaster.json and go through the import flow

This PR adds a short note to both quickstart pages so it's easier to choose the right path.

Closes #1247

[cb-heimdall]

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