docs: add Authentik documentation (#124)

* Add Authentik documentation

* Adjust description for Authentik.

* Formatting adjustments

pages/docs/configuration/authentication/OAuth2-OIDC/authentik.mdx

@@ -0,0 +1,53 @@
+---
+title: Authentik
+description: Learn how to configure LibreChat to use Authentik for user authentication.
+---
+
+# Authentik
+
+1. **Access Authentik Admin Interface:**
+- Open the Authentik Admin Interface in your browser. Can be found at a URL such as: `https://authentik.example.com/if/admin/#/administration/overview`.
+> We will use `https://authentik.example.com` as an example URL. Replace this with the URL of your Authentik instance.
+
+2. **Create a new Application and Provider using the wizard:**
+- Click on the Applications tab in the left sidebar and click on Applications again.
+- At the top of the page you should see a button that says `Create with Wizard`. Click on it.
+> Note: You can also create an application and provider manually just be sure to link them afterwards.
+- You can name the application whatever you want. For this example, we will name it `LibreChat` and click next.
+- Choose the `OAuth2/OIDC` provider and click next.
+- Choose your authentication and authorization flows.
+- Scroll down and take note of the `Client ID` and `Client Secret`. You will need these later.
+- Under Advanced protocol settings change Subject mode to `Based on the User's Email`.
+- Click Submit.
+- Add the new application you created to an Outpost.
+> Note: You should also apply any policies for access control that you want to apply to LibreChat at this point.
+
+3. **Gather Information for .env:**
+- You will need the following information from Authentik:
+  - `Client ID`
+  - `Client Secret`
+  - `OpenID Configuration URL`
+  > All of these can be found by clicking on the provider you just created.
+
+3. **Configure LibreChat:**
+- Open the `.env` file and add the following variables:
+```bash filename=".env"
+OPENID_ISSUER=https://authentik.example.com/application/o/librechat/.well-known/openid-configuration
+OPENID_CLIENT_ID=[YourClientID]
+OPENID_CLIENT_SECRET=[YourClientSecret]
+OPENID_SESSION_SECRET=[JustGenerateARandomSessionSecret]
+OPENID_CALLBACK_URL=/oauth/openid/callback
+OPENID_SCOPE=openid profile email
+# Optional customization below
+OPENID_BUTTON_LABEL=Login with Authentik
+OPENID_IMAGE_URL=https://cdn.jsdelivr.net/gh/selfhst/icons/png/authentik.png
+```
+> Note: Make sure nothing is wrapped in quotes in your .env and you have allowed social login.
+
+4. **Check Configuration:**
+- Restart LibreChat to apply the changes.
+- Open an Icognito window and navigate to your LibreChat instance.
+- Underneath the form login there should be a new button that says `Login with Authentik`.
+- You should be redirected to Authentik to login.
+- After logging in you should be redirected back to LibreChat and be logged in.
+    - If you are not redirected back to LibreChat, check Authentik logs for any errors.