docs: add rancher integration

docs/integrations/as-sp/rancher/index.md

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+# Rancher Integration
+
+Rancher can authenticate against passbook using SAML.
+
+## Preparation
+
+The following placeholders will be used:
+
+-   `rancher.company` is the FQDN of the Rancher Install
+-   `passbook.company` is the FQDN of the passbook Install
+
+Create an application in passbook and note the slug, as this will be used later. Create a SAML Provider with the following Parameters:
+
+-   `ACS URL`: https://rancher.company/v1-saml/adfs/saml/acs
+-   `Audience`: https://rancher.company/v1-saml/adfs/saml/metadata
+-   `Issuer`: passbook
+
+You can of course use a custom Signing Certificate, and adjust the Assertion Length.
+
+## Rancher
+
+![aaa](./rancher.png)