docs: update composition getting started guide with yaml+cel example

[jbw976]

This PR adds function-kro to the getting started with composition guide.

The example has been tested manually and the composed resources look healthy:

❯ k get pkg
NAME                                                         INSTALLED   HEALTHY   PACKAGE                                                     AGE
function.pkg.crossplane.io/crossplane-contrib-function-kro   True        True      xpkg.crossplane.io/crossplane-contrib/function-kro:v0.1.0   113s

❯ crossplane beta trace app.example.crossplane.io/my-app
NAME                                          SYNCED   READY   STATUS
App/my-app (default)                          True     True    Available
├─ Deployment/my-app-730ec43d11d3 (default)   -        -
└─ Service/my-app-43f18e329656 (default)      -        -

❯ kubectl get deploy,service -l example.crossplane.io/app=my-app
NAME                                  READY   UP-TO-DATE   AVAILABLE   AGE
deployment.apps/my-app-730ec43d11d3   2/2     2            2           28s

NAME                          TYPE        CLUSTER-IP      EXTERNAL-IP   PORT(S)    AGE
service/my-app-43f18e329656   ClusterIP   10.96.233.188   <none>        8080/TCP   28s

• sync approved content to v2.2 content dir also

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[haarchri]

Thanks - nice to add YAML + CEL