Add VoidAuth OIDC documentation

docs/administration/oidc/voidauth.md

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+---
+title: OIDC with VoidAuth
+description: Wire up SSO to VoidAuth
+---
+
+# OIDC with VoidAuth
+
+[VoidAuth](https://voidauth.app/) is an open-source SSO authentication and user management provider that stands guard in front of your self-hosted application. Before starting, read the [OIDC Setup overview](index.md), as it covers the RomM-side settings common to every provider.
+
+## 1. Prerequisites
+
+VoidAuth installed and running via their [self-hosted deployment docs](https://voidauth.app/#/?id=quick-start). We will use the APP_URL provided in the compose.yml exemple for reference in this documentation : https://auth.example.com
+Your RomM instance will be referenced as https://demo.romm.app .
+
+## 2. Create a new app
+
+Login as an admin in VoidAuth web interface. Create a new OIDC Apps  (e.g. `RomM`). 
+
+- **Name**: `RomM`
+- **Home Page URL**: `https://demo.romm.app`
+- **Logo URL**: `https://github.com/HydrelioxGitHub/romm-docs/blob/main/.github/resources/isotipo.png?raw=true`
+- **Group**: You could add a group that the user must belong to get access to your RomM instance. If left empty, any user created in your VoidAuth instance will be allowed.
+- **Skip Consent** and **MFA Required** : This options could be checked or not as you wish.
+- **Client ID**: Generate an ID using the button
+- **Auth Method**: `Client Secred Basic`
+- **Client Secret**: Generate a secret using the button
+- **Redirect URLs**: add `https://demo.romm.app/api/oauth/openid`
+- **Response Types**: check `code`
+- **Grant Types**: check `authorization_code` and `refresh_token`
+- **Post Logout URL**: `https://demo.romm.app/`
+
+Don't forget to click on the `Create` button to valid your app.
+
+## 3. Configure
+
+```yaml
+environment:
+    - OIDC_ENABLED=true
+    - OIDC_PROVIDER=VoidAuth
+    - OIDC_CLIENT_ID=<from VoidAuth>
+    - OIDC_CLIENT_SECRET=<from VoidAuth>
+    - OIDC_REDIRECT_URI=https://demo.romm.app/api/oauth/openid
+    - OIDC_SERVER_APPLICATION_URL=https://auth.example.com
+    - OIDC_SERVER_METADATA_URL=https://auth.example.com/oidc/.well-known/openid-configuration
+    - ROMM_BASE_URL=https://demo.romm.app
+```
+
+VoidAuth's OIDC discovery URL can be found at the top of your OIDC App page, by clicking on OIDC Endpoints.
+
+For role mapping from VoidAuth, see [OIDC Setup → Role mapping](index.md#role-mapping).
+
+## 4. Test
+
+Restart, navigate to `/login` and click the **Login with VoidAuth** button. You're redirected to VoidAuth → authenticate → bounced back and signed in!
+
+If it doesn't work, head to [Authentication Troubleshooting](../../troubleshooting/authentication.md).