docs: clarify template environment configuration

[Danielkis97]

Summary

Improves the template environment files with clearer comments for new developers.

The existing templates only listed the required variables without explaining what they are used for. This adds short guidance for:

• project name
• OnchainKit API key
• Mini App public URL
• Base Sepolia development note

Fixes #2644

Testing

• git diff --check

[vercel]

Someone is attempting to deploy a commit to the Coinbase Team on Vercel.

A member of the Team first needs to authorize it.

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	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