First attempt at some documentation

docs/setup/oidc.md

@@ -0,0 +1,34 @@
+# OpenID Connect
+
+You can set up RustiCal with an OpenID Connect identity provider
+
+## Example: Authelia
+
+```toml title="RustiCal configuration"
+[frontend.oidc]
+name = "Authelia"
+issuer = "https://auth.example.com"
+client_id = "rustical"
+client_secret = "secret..."
+claim_userid = "preferred_username"  # (1)!
+scopes = ["openid", "profile", "groups"]
+require_group = "app/rustical"  # (2)!
+allow_sign_up = false
+```
+
+1. Can be either `preferred_username` or `sub`
+2. Optional: You can require a user to be in a certain group to use RustiCal
+
+```yaml title="Authelia configuration"
+identity_providers:
+  oidc:
+    clients:
+      - client_id: rustical
+        client_secret: secret...
+        public: false
+        consent_mode: implicit
+        scopes: [openid, profile, groups]
+        token_endpoint_auth_method: client_secret_basic
+        redirect_uris:
+          - https://rustical.example.com/frontend/login/oidc/callback
+```