[feat] Add E-commerce Domain Example to domains.md

[operfildoluiz]

This PR adds an E-commerce section to nbs/domains.md, following the same format used for Restaurants.

The section explains how online stores can use llms.txt to expose structured product information (e.g., SKUs, specs, nutrition data, variant rules, pricing tiers, subscription terms) so LLMs can answer product and purchasing questions accurately.

It includes:

• A short explanation of why e-commerce benefits from it
• Guidance on structuring the document around product data rather than navigation
• A complete llms.txt example for a protein bar store (I called it FooBar just for the sake of the joke)
• A sample product resource demonstrating structured, machine-readable product details

This expands the domain coverage to include e-commerce use cases.