Missing warning about reliability of fallback RPC URL in build-app guide

## Description
In Step 4 of the "Build an app on Base" guide, the docs suggest using
`https://base-sepolia-rpc.publicnode.com` as a fallback RPC if the
default endpoint is unreachable — but no warning is given about its
limitations.

## Page URL
https://docs.base.org/get-started/build-app

## Problem
The page already warns about the main RPC endpoint:
"For production apps, use a dedicated RPC provider."
But this warning is absent for the publicnode fallback URL, which is also
rate-limited and unsuitable for production.

## Suggested fix
Add a brief note after the publicnode fallback URL, similar to the
existing warning, clarifying it is also rate-limited and for testing only.