docs(kubernetes): add --devel for Pangolin Helm pre-release installs

## Description

The Pangolin Helm chart is currently intentionally published as a pre-release (`0.1.0-alpha.1`).

Helm excludes pre-release chart versions from normal searches and installs unless `--devel` is specified or the chart version is explicitly provided.

The current Kubernetes Helm documentation uses:

```bash
helm search repo fossorial
```

and:

```bash
helm install my-pangolin fossorial/pangolin
```

As a result, on a clean Helm installation the Pangolin chart is not shown by the search and the installation fails because there is currently no stable Pangolin chart release.

## Proposed change

Update the Pangolin Helm documentation to use `--devel` while the chart remains in alpha:

```bash
helm search repo fossorial --devel
```

and:

```bash
helm install my-pangolin fossorial/pangolin --devel
```

Alternatively, document that users can explicitly pin the current pre-release version:

```bash
helm install my-pangolin fossorial/pangolin \
--version 0.1.0-alpha.1
```

This should only affect the Pangolin instructions. The chart should remain marked as a pre-release until it is actually ready for a stable release.

## Related issue

Transferred from:

https://github.com/fosrl/helm-charts/issues/21