Repository: jupyterhub/zero-to-jupyterhub-k8s

Issue:
Update values schema to allow key `enabled`
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### Proposed change
<!-- Use this section to describe the feature you'd like to be added. -->

Update values schema to allow key `enabled` so the z2jh helm chart can be included as a dependency in other helm charts and be enabled using the preffered syntax

Usually, when including a chart as a helm dependency, the chart can be turned on and off by setting the charts `enabled` key to `true` or `false`.

This is the preffered method according to the [helm docs](https://helm.sh/docs/chart_best_practices/dependencies/#conditions-and-tags)

For example, if jupyterhub is a helm dependency, then it would look like this in the parent chart's `values.yaml` file:
```yaml
jupyterhub:
    enabled: true
    hub:
        ...
```

As of now, if you try to include the `enabled` key in a values.yaml, the jupyterhub values scheme will throw a validation error:
```
failed to upgrade release '<name>' from chart '<chart>': values don't meet the specifications of the schema(s) in the following chart(s):
jupyterhub:
- (root): Additional property enabled is not allowed
```

The values schema should be updated to allow for the enabled key so z2jh can be used as a dependency


### Alternative options
<!-- Use this section to describe alternative options and why you've decided on the proposed feature above. -->

Unsure

### Who would use this feature?
<!-- Describe who would benefit from using this feature. -->

Users who want to incorporate jupyterhub as a dependency in their own helm charts


### (Optional): Suggest a solution
<!-- Describe what you think needs to be done. Doing that is an excellent first step to get the feature implemented. -->

Solution in PR #3162



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Resolving patch:
diff --git a/docs/source/conf.py b/docs/source/conf.py
--- a/docs/source/conf.py
+++ b/docs/source/conf.py
@@ -217,6 +217,7 @@ def parse_schema(d, md=[], depth=0, pre=""):
     "https://your-domain-name.com",  # example
     "https://kubernetes.io/docs/tutorials/kubernetes-basics/",  # works
     "https://cloud.ibm.com/kubernetes/catalog/create",  # works
+    "https://github.com/traefik/traefik/blob/HEAD/CHANGELOG.md",  # works
     "https://portal.azure.com",  # sign-in redirect noise
     "https://console.cloud.google.com",  # sign-in redirect noise
     "https://console.developers.google.com",  # sign-in redirect noise


Test patch: