💡 [REQUEST] - Add end-to-end quickstart to `README.md` for new users

### Summary

At the time of writing, `README.md` has an installation and basic usage section, but no actual end-to-end example based on a simple application or service actually running on GCP.

Can we add such an example so new users can run the commands and experiment first-hand what the SLO Generator does?

### Basic Example

Add an example over a simple service running in Cloud Run, deployed from an image available on a public registry. For example an application that fails on purpose 20% of the time. Then define an SLO with a 95% target over 28 days and confirm the output shows up as expected with alerts.

### Screenshots

```bash
```

### Drawbacks

None identified. This will help new users onboard faster.

### Unresolved questions

_No response_

### Code of Conduct

- [X] I agree to follow this project's Code of Conduct