fix(docs): add rate-limit warning to publicnode fallback RPC URL

[schoolkamsergj]

Summary

Fixes #1356

The build-app guide suggests https://base-sepolia-rpc.publicnode.com as a fallback RPC endpoint if https://sepolia.base.org is unreachable, but includes no warning about its limitations.

The same page already carries this note for the primary endpoint:

"For production apps, use a dedicated RPC provider."

The fallback URL is equally rate-limited and unsuitable for production, so the missing warning creates a misleading inconsistency.

Change

Extended the existing <Note> in Step 4 to add one sentence:

This endpoint is also rate-limited and not recommended for production apps.

No other content was modified.

Before / After

Before:

If `https://sepolia.base.org` is unreachable, use an alternative public endpoint
such as `https://base-sepolia-rpc.publicnode.com`. For production apps, use a
dedicated RPC provider.

After:

If `https://sepolia.base.org` is unreachable, use an alternative public endpoint
such as `https://base-sepolia-rpc.publicnode.com`. This endpoint is also
rate-limited and not recommended for production apps. For production apps, use a
dedicated RPC provider.

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/2	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2