docs(oidc): add back-channel logout (#9034)

# Which Problems Are Solved

OIDC Back-Channel Logout released with
[V2.65.0](https://github.com/zitadel/zitadel/releases/tag/v2.65.0) were
not yet documented

# How the Problems Are Solved

- Added small guide and description
- Updated claims (added `sid` and `events`)

# Additional Changes

None

# Additional Context

relates to https://github.com/zitadel/zitadel/issues/8467

docs/sidebars.js

@@ -258,6 +258,7 @@ module.exports = {
           ],
         },
         "guides/integrate/token-exchange",
+        "guides/integrate/back-channel-logout",
         {
           type: "category",
           label: "Service Users",