[BUG][opensearch-dashboards] Add opensearchURL to values.yaml and document it

[schroedermatthias]

Describe the bug
In the deployment.yaml the value .Values.opensearchURL is referenced:

helm-charts/charts/opensearch-dashboards/templates/deployment.yaml

Line 120 in 4ca8bfd

{{- if .Values.opensearchURL }}

It is not provided in the values.yaml. It also should be made clear, what's the purpose and difference of opensearchURL and opensearchHosts.
Additionaly, a hint would be nice, that opensearchHosts sets an environment variable which overrules the directive
opensearch.hosts: ... in /usr/share/opensearch-dashboards/config/opensearch_dashboards.yaml.

To Reproduce
Steps to reproduce the behavior:

1. Install helm chart in version 2.20.0

Expected behavior
opensearchURL and opensearchHosts should be added to the values.yaml and documented.

Chart Name
opensearch-dashboards

[prudhvigodithi]

[Triage]
Hey @schroedermatthias we should find the background of OPENSEARCH_URL and if not used anymore we should remove from the chart, coming from an example here https://github.com/opensearch-project/opensearch-build/blob/main/docker/release/dockercomposefiles/docker-compose-2.x.yml#L59C7-L59C23 I see the OPENSEARCH_HOSTS is one that is used.