website/docs: add new SSF provider docs (#13102)

* website/docs/add-secure-apps/providers/sff/index.md

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* backchannel info

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Co-authored-by: Tana M Berry <tana@goauthentik.com>

website/docs/add-secure-apps/providers/ssf/create-ssf-provider.md

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+---
+title: Configure an SSF provider
+---
+
+The workflow to implement an SSF provider as a [backchannel provider](../../applications/manage_apps#backchannel-providers) for an application/provider pair is as follows:
+
+1. Create the SSF provider (which serves as the backchannel provider).
+2. Create an OIDC provider (which serves as the protocol provider for the application).
+3. Create the application, and assign both the OIDC provider and the SSF provider.
+
+## Create the SSF provider
+
+1. Log in to authentik as an admin, and in the Admin interface navigate to **Applications -> Providers**.
+
+2. Click **Create**.
+
+3. In the modal, select the **Provider Type** of **SSF**, and then click **Next**.
+
+4. On the **New provider** page, provide the configuration settings. Be sure to select a **Signing Key**.
+
+5. Click **Finish** to create and save the provider.
+
+## Create the OIDC provider
+
+1. Log in to authentik as an admin, and in the Admin interface navigate to **Applications -> Providers**.
+
+2. Click **Create**.
+
+3. In the modal, select the **Provider Type** of **OIDC**, and then click **Next**.
+
+4. Define the settings for the provider, and then click **Finish** to save the new provider.
+
+## Create the application
+
+1. Log in to authentik as an admin, and in the Admin interface navigate to **Applications -> Applications**.
+
+2. Click **Create**.
+
+3. Define the settings for the application:
+
+    - **Name**: define a descriptive name ofr the application.
+    - **Slug**: optionally define the internal application name used in URLs.
+    - **Group**: optionally select a group that you want to have access to this application.
+    - **Provider**: select the OIDC provider that you created.
+    - **Backchannel Providers**: select the SSF provider you created.
+    - **Policy engine mode**: define policy-based access.
+    - **UI Settings**: optionally define a launch URL, an icon, and other UI elements.
+
+4. Click **Create** to save the new application.
+
+The new application, with its OIDC provider and the backchannel SFF provider, should now appear in your list of Applications.