doc: have units-description.md include appVersion for Helm charts

Today for Helm-based units units-description.md gives us only the Helm chart version. In many cases the chart version does not tell which is the version of the application.

E.g. for Kyverno, for chart version 3.5.1, the Kyverno version is v1.15.1.

We would benefit from having this information in this file:

• for end users
• for us when looking at MRs that do Helm chart version increments

To achieve this, tools/generate_units_documentation.py could be extended to run helm show <repo>/<chart> or helm show oci://... for the chart, and retrieve the appVersion field from there. And it would include this information side-by-side with the Helm chart version in its output.