Documentation Request: Improve template environment configuration for Base Sepolia development

### Is your documentation request related to a problem? Please describe.

## Problem

The current `.template.env` configuration inside the Next.js template is very minimal and may be confusing for developers setting up local Base Sepolia environments.

## Proposed Improvement

Improve the template environment file by:

* adding descriptive comments
* including a Base Sepolia RPC example
* making onboarding clearer for developers

## Benefit

This would improve the developer experience and reduce setup friction for developers using the starter template.