docs: Inconsistent API endpoint URLs across x402 docs

[Saiful-15931]

Problem

The x402 Overview page lists the endpoint as:
https://llm.opengradient.ai

But all code examples use a different URL:
https://llmogevm.opengradient.ai

This inconsistency is dangerous because:

• Developers don't know which URL to use
• Wrong URL = failed payment transactions
• Real money ($OPG tokens) could be lost

Where

• Overview: docs.opengradient.ai/developers/x402/
• Examples: docs.opengradient.ai/developers/x402/examples.html

Requested Fix

Please clarify which URL is correct and update
all docs to use the same endpoint