docs: add Jellyfin Integration Guide (#51)

docs/jellyfin.md

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+# Jellyfin SSO Integration Guide
+
+> Due to the current limitations of the Jellyfin SSO plugin, this integration will only work in a browser. When tested, the Jellyfin app did not work and displayed an error, even when custom menu buttons were created.
+
+> To view the original references and a full list of capabilities, please visit the [Jellyfin SSO OpenID Section](https://github.com/9p4/jellyfin-plugin-sso?tab=readme-ov-file#openid).
+
+### Requirements
+- [Jellyfin Server](https://jellyfin.org/downloads/server)
+- [Jellyfin SSO Plugin](https://github.com/9p4/jellyfin-plugin-sso)
+- HTTPS connection to your Jellyfin server
+
+### OIDC - Pocket ID Setup
+To start, we need to create a new SSO resource in our Jellyfin application.
+
+> Replace the `JELLYFINDOMAIN` and `PROVIDER` elements in the URL.
+
+1. Log into the admin panel, and go to OIDC Clients -> Add OIDC Client.
+2. **Name**: Jellyfin (or any name you prefer)
+3. **Callback URL**: `https://JELLYFINDOMAIN.com/sso/OID/redirect/PROVIDER`
+4. For this example, we’ll be using the provider named "test_resource."
+5. Click **Save**. Keep the page open, as we will need the OID client ID and OID secret.
+
+### OIDC Client - Jellyfin SSO Resource
+
+1. Visit the plugin page (<i>Administration Dashboard -> My Plugins -> SSO-Auth</i>).
+2. Enter the <i>OID Provider Name (we used "test_resource" as our name in the callback URL), Open ID, OID Secret, and mark it as enabled.</i>
+3. The following steps are optional based on your needs. In this guide, we’ll be managing only regular users, not admins.
+
+![img.png](imgs/jelly_fin_img.png)
+
+> To manage user access through groups, follow steps **4, 5, and 6**. Otherwise, leave it blank and skip to step 7.
+
+![img2.png](imgs/jelly_fin_img2.png)
+
+4. Under <i>Roles</i>, type the name of the group you want to use. **Note:** This must be the group name, not the label. Double-check in Pocket ID, as an incorrect name will lock users out.
+5. Skip every field until you reach the **Role Claim** field, and type `groups`.
+   > This step is crucial if you want to manage users through groups.
+6. Repeat the above step under **Request Additional Scopes**. This will pull the group scope during the sign-in process; otherwise, the previous steps won’t work.
+
+![img3.png](imgs/jelly_fin_img3.png)
+
+7. Skip the remaining fields until you reach **Scheme Override**. Enter `https` here. If omitted, it will attempt to use HTTP first, which will break as WebAuthn requires an HTTPS connection.
+8. Click **Save** and restart Jellyfin.
+
+### Optional Step - Custom Home Button
+Follow the [guide to create a login button on the login page](https://github.com/9p4/jellyfin-plugin-sso?tab=readme-ov-file#creating-a-login-button-on-the-main-page) to add a custom button on your sign-in page. This step is optional, as you could also provide the sign-in URL via a bookmark or other means.
+
+### Signing into Your Jellyfin Instance
+Done! You have successfully set up SSO for your Jellyfin instance using Pocket ID.
+
+> **Note:** Sometimes there may be a brief delay when using the custom menu option. This is related to the Jellyfin plugin and not Pocket ID.
+
+If your users already have accounts, as long as their Pocket ID username matches their Jellyfin ID, they will be logged in automatically. Otherwise, a new user will be created with access to all of your folders. Of course, you can modify this in your configuration as desired.
+
+This setup will only work if sign-in is performed using the `https://jellyfin.example.com/sso/OID/start/PROVIDER` URL. This URL initiates the SSO plugin and applies all the configurations we completed above.