docs: Extend the documentation for OpenSearch Dashboards

[siegfriedweber]

Description

The documentation does not provide the necessary information to deploy OpenSearch Dashboards and connect it to an OpenSearch instance deployed by the Stackable OpenSearch operator. This pull requests tries to add the missing information.

• Fix a typo in the admin password on the First Steps page
  ⇒ This fix is a reason to back-port this pull request to the release-25.11 branch.
• Add usage information about OpenSearch Dashboards to the First Steps page
• Enable TLS in OpenSearch Dashboards (to be secure by default)
• Adjust the opensearch-dashboards integration test

Part of #1

Backport to SDP 25.11: #78

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

Author

• Changes are OpenShift compatible
• Integration tests passed (for non trivial changes)
• Links to generated (nightly) docs added
  • https://docs.stackable.tech/home/nightly/opensearch/getting_started/first_steps/#_using_opensearch_dashboards
  • https://docs.stackable.tech/home/nightly/opensearch/usage-guide/opensearch-dashboards/

Reviewer

• Code contains useful comments
• Code contains useful logging statements
• (Integration-)Test cases added
• Documentation added or updated. Follows the style guide.
• Changelog updated
• Cargo.toml only contains references to git tags (not specific commits or branches)

Acceptance

• Feature Tracker has been updated
• Proper release label has been added
• Links to generated (nightly) docs added
• Release note snippet added
• Add type/deprecation label & add to the deprecation schedule
• Add type/experimental label & add to the experimental features tracker

[labrenbe]

lgtm