Document Rancher group permissions workaround

[rb3ckers]

Publish the known Rancher group-permission issue and generic-OIDC workaround for existing SUSE Observability installations. Link it from the Rancher authentication instructions and list Rancher in the authentication options.

Targets main independently of #433; no unreleased chart option or version placeholder. Tracks https://github.com/StackVista/helm-charts-internal/issues/233.

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[LouisLotter]

P2 — Preserve session lifetime in the workaround (troubleshooting.adoc:68).

With no explicit stackstate.authentication.sessionLifetime, switching from rancher to generic oidc changes the chart default from 16 hours to seven days. I confirmed this with a focused render of the authentication template from https://github.com/StackVista/helm-charts-internal/pull/236.

Add sessionLifetime: 16h under stackstate.authentication for installations using the default, and tell users to retain any explicit custom value. This keeps the workaround from silently extending the session lifetime. The same applies to the workaround in #433.