docs: add IDS6 documentation

docs/docs/providers/duende-identity-server6.md

@@ -0,0 +1,53 @@
+---
+id: duende-identityserver6
+title: DuendeIdentityServer6
+---
+
+## Documentation
+
+https://docs.duendesoftware.com/identityserver/v6
+
+## Options
+
+The **DuendeIdentityServer6 Provider** comes with a set of default options:
+
+- [DuendeIdentityServer6 Provider options](https://github.com/nextauthjs/next-auth/blob/v4/packages/next-auth/src/providers/duende-identity-server6.ts)
+
+You can override any of the options to suit your own use case.
+
+## Example
+
+```js
+import DuendeIDS6Provider from "next-auth/providers/duende-identity-server6"
+
+...
+providers: [
+  DuendeIDS6Provider({
+    clientId: process.env.DUENDE_IDS6_ID,
+    clientSecret: process.env.DUENDE_IDS6_SECRET,
+    issuer: process.env.DUENDE_IDS6_ISSUER,
+  })
+]
+...
+```
+
+## Demo IdentityServer
+
+The configuration below is for the demo server at https://demo.duendesoftware.com/
+
+If you want to try it out, you can copy and paste the configuration below.
+
+You can sign in to the demo service with either <b>bob/bob</b> or <b>alice/alice</b>.
+
+```js
+import DuendeIDS6Provider from "next-auth/providers/duende-identity-server6"
+...
+providers: [
+  DuendeIDS6Provider({
+    clientId: "interactive.confidential",
+    clientSecret: "secret",
+    issuer: "https://demo.duendesoftware.com",
+  })
+]
+...
+```