Improve issue_asset example: added detailed comments, installation guide, and step-by-step explanation

[otaviovin]

This Pull Request improves the examples/issue_asset.py example by making it more educational, consistent with the existing examples, and aligned with the official Stellar Python SDK documentation.

Summary of Changes:

• Added a comprehensive header docstring describing the purpose and workflow of asset issuance on the Stellar Testnet.
• Included installation and execution instructions for running the example (pip install stellar-sdk and python issue_asset.py).
• Added detailed step-by-step comments explaining how to create, trust, and distribute a custom asset.
• Documented the expected output to help developers verify successful trustline creation and asset transfer.
• Improved readability, formatting, and consistency with other examples in the repository.
• Added guidance and best practices about separating issuing and distribution accounts for better security.

Purpose:

This enhancement helps developers understand how to:

1. Connect to the Stellar Testnet using the Python SDK.
2. Create and define a new custom asset with the Asset class.
3. Establish a trustline between the issuing and distribution accounts.
4. Send the newly issued asset from the issuer to the distributor.
5. Follow Stellar’s recommended asset issuance model for secure token management.
   By following this example, developers can easily learn the full process of issuing, distributing, and managing assets on the Stellar network.

References:

• Stellar Docs – Issuing Assets: https://developers.stellar.org/docs/issuing-assets/
• Stellar Token Design & Security Model: https://developers.stellar.org/docs/tokens
• Stellar SDK for Python: https://github.com/StellarCN/py-stellar-base

These changes make the issue_asset.py example self-contained, easy to follow, and consistent with other educational examples in the py-stellar-base repository.
It serves as a practical guide for developers learning how to create and distribute custom assets securely using the Stellar Python SDK on the Testnet.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.97%. Comparing base (fba85a5) to head (93bf895).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

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  Coverage   97.97%   97.97%           
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  Files         262      262           
  Lines       14799    14799           
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  Hits        14499    14499           
  Misses        300      300           

Flag	Coverage Δ
unittests	97.97% <ø> (ø)

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