docs: add mock usdt testing guide for shasta testnet

[goldman0020]

This PR adds a practical guide for testing TRC-20 token interactions on Shasta testnet using a pre-deployed Mock USDT contract.

Contract Details:

• Address: TWy36p1Ghn5rqRcdink7XPWyRakwtxjpie
• Name: Mock USDT
• Symbol: mUSDT
• Decimals: 18
• Source Code: Included in the documentation

Why this is useful:

• No need to deploy your own contract
• Saves time and TRX for developers
• Provides real working examples

Checklist:

• Tested on Shasta testnet
• Full Solidity source code included
• Complete test script provided
• Error handling and security warnings included
• No duplication of existing documentation

[goldman0020]

@Stan202310 Thanks for the link – it's a great full-stack DApp template.

The goal of my PR is complementary: a minimal, copy-paste snippet for developers to quickly verify their TronWeb setup and test TRC-20 logic (balanceOf, transfer, mint) without the overhead of a full React/Vite project. It's focused purely on the "TronWeb logic" itself.

I believe having this concise example in docs/examples/ alongside the quickstart would lower the barrier for new developers – a fast way to test core concepts before building a full DApp.

Happy to adjust anything to better fit the contribution guidelines. Let me know your thoughts!