[backport v2.15.1] Support disabling of local auth provider - link to docs

[rancher-ui-project-bot]

This is a backport issue for #18144, automatically created via GitHub Actions workflow initiated by @richard-cox

Original issue body:

• In places that we describe the impact and talk about requirements we should link to the official docs
• The URL will be made available as part of Local Auth is disabled upon External Auth provider setup rancher-product-docs#1227

[richard-cox]

blocked on rancher/rancher-product-docs#1227