Add Quick Start guide for wallet balance check

[BehllyTamar191]

What this PR does

Adds a beginner-friendly Quick Start section to the README showing how to:

• Connect PyCardano to the Blockfrost testnet
• Check a wallet's balance

Why

This helps new developers get started with PyCardano quickly without needing to dig through examples or full documentation.

How to test

1. Follow the steps in the Quick Start section.
2. Run the example Python code with your testnet Blockfrost API key.
3. Verify that your wallet balance is returned correctly.

[BehllyTamar191]

Hi @Python-Cardano maintainers 👋
This PR adds a Quick Start example for checking wallet balance on Blockfrost testnet.
It’s beginner-friendly and purely documentation. Would love your review when you have a chance. Thanks! 🙏

[cffls]

The example provided in the diff seems incomplete. Could you double check your commit? Thanks!