docs: Add practical examples directory

[vincent067]

Hi there! 👋

First off, thanks for building such an awesome library! execa has been a joy to use in my projects.

This PR adds a new examples/ directory with practical, real-world code samples. I noticed that while the documentation is excellent, there's a bit of a learning curve when transitioning from reading the docs to actually implementing common use cases. These examples aim to bridge that gap! 🚀

What's included

File	Description
build-script.js	Running build commands with error handling and logging
git-helpers.js	Common Git operations using execa
parallel-tasks.js	Executing multiple commands concurrently
stream-processing.js	Working with stdout/stderr streams

Why this helps

• New users can see patterns they can adapt immediately
• Common workflows are demonstrated with best practices
• Less friction from "how do I...?" to "I get it!"

I'm happy to adjust the examples based on your feedback—whether that's adding more comments, changing the coding style, or including additional use cases. Just let me know! 😊

Thanks for considering this contribution! 🙏