docs: improve template environment configuration for Base Sepolia development

[Hobie1Kenobi]

What changed? Why?

Improves .template.env\ for the \onchainkit-nextjs\ starter template (fixes #2644).

Changes:

• Added header with copy-to-.env.local\ instructions
• Documented \NEXT_PUBLIC_PROJECT_NAME\ and \NEXT_PUBLIC_ONCHAINKIT_API_KEY\
• Added Base Sepolia / Base mainnet RPC examples and onboarding notes
• Included \NEXT_PUBLIC_RPC_URL\ default pointing at the public Base Sepolia endpoint

Benefit

Reduces setup friction for developers targeting Base Sepolia testnet with clearer env var guidance.

How has it been tested?

• Verified valid .env\ syntax
• Confirmed RPC URLs match official Base documentation

Payout address (Base): \

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[cb-heimdall]

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[Hobie1Kenobi]

Thanks for the issue — quick note on overlap:

#2651 already adds comprehensive commented guidance to templates/onchainkit-nextjs/.template.env (and minikit-nextjs), which covers the intent of #2644.

This PR (#2663) largely duplicates the onchainkit-nextjs documentation; the only extra delta here is an explicit default NEXT_PUBLIC_RPC_URL=https://sepolia.base.org.

Closing to reduce duplicate review load. If maintainers want that explicit RPC default, I'm happy to open a tiny follow-up on top of #2651 after it lands.

(Eth payout on file in PR body if useful for record-keeping.)

[Hobie1Kenobi]

Closing as duplicate of #2651 for onchainkit-nextjs; see comment above.