docs: Suggestion to Include a Sandbox OIDC Provider for Easier Onboarding

*Description*:
As a new user, I found it difficult to follow the OIDC Authentication documentation because it requires setting up an external OIDC provider (e.g., Google, Auth0, etc.). The additional setup steps on a separate platform disrupted my flow and made it harder to test OIDC authentication quickly.

To improve the developer experience, I recommend including a sandbox OIDC provider that users can use out-of-the-box without having to register an external OIDC application. This would significantly lower the barrier to testing OIDC authentication in Envoy Gateway.

- [ ] Provide a pre-configured OIDC sandbox as part of the documentation.
- [ ] Offer an example manifest that deploys a local OIDC provider alongside Envoy Gateway in Kubernetes.
- [ ] Include test credentials so users can authenticate without registering an application externally.
- [ ] Modify the existing documentation to first guide users through the sandbox setup before introducing external OIDC providers.

Page: https://gateway.envoyproxy.io/docs/tasks/security/oidc/