Add README for attributions example

[codegen-sh]

This PR adds a comprehensive README for the attributions example, following the structure and content of the attributions tutorial in the docs.

The README includes:

• Overview of the attribution extension
• How the example works
• Instructions for running the example
• Code walkthrough with explanations
• Key insights from using the attribution extension
• Link to further reading

This makes the example more accessible and helps users understand how to use the attribution extension to analyze AI contributions in their codebase.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

codegen-bot seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}