sources/oauth: add Sign in with Apple (#1635)

* sources/oauth: add apple sign in support

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* website/docs: apple sign in docs

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* website/docs: fix missing apple in sidebar

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

* sources/oauth: add fallback values for name and slug

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

website/docs/integrations/sources/apple/index.md

@@ -0,0 +1,67 @@
+---
+title: Apple
+---
+
+Allows users to authenticate using their Apple ID.
+
+## Preparation
+
+:::warning
+An Apple developer account is required for this.
+:::
+
+The following placeholders will be used:
+
+- `authentik.company` is the FQDN of the authentik install.
+
+## Apple
+
+1. Log into your Apple developer account, and navigate to **Certificates, IDs & Profiles**, then click **Identifiers** in the sidebar.
+2. Register a new Identifier with the type of **App IDs**, and the subtype **App**.
+3. Choose a name that users will recognise for the **Description** field.
+4. For your bundle ID, use the reverse domain of authentik, in this case `company.authentik`.
+5. Scroll down the list of capabilities, and check the box next to **Sign In with Apple**.
+6. At the top, click **Continue** and **Register**.
+
+![](app_id.png)
+
+7. Register another new Identifier with the type of **Services IDs**.
+8. Again, choose the same name as above for your **Description** field.
+9. Use the same identifier as above, but add a suffix like `signin` or `oauth`, as identifiers are unique.
+10. At the top, click **Continue** and **Register**.
+
+![](service_id.png)
+
+11. Once back at the overview list, click on the just-created Identifier.
+12. Enable the checkbox next to **Sign In with Apple**, and click **Configure**
+13. Under domains, enter `authentik.company`.
+14. Under **Return URLs**, enter `https://authentik.company/source/oauth/callback/apple/`.
+
+![](app_service_config.png)
+
+15. Click on **Keys** in the sidebar. Register a new Key with any name, and select **Sign in with Apple**.
+16. Click on **Configure**, and select the App ID you've created above.
+17. At the top, click **Save**, **Continue** and **Register**.
+18. Download the Key file and note the **Key ID**.
+
+![](key.png)
+
+19. Note the Team ID, visible at the top of the page.
+
+## authentik
+
+20. Under _Resources -> Sources_ Click **Create Apple OAuth Source**
+
+21. **Name**: `Apple`
+22. **Slug**: `apple`
+23. **Consumer Key:** The identifier from step 9, then `;`, then your Team ID from step 19, then `;`, then the Key ID from step 18.
+
+    Example: `io.goauthentik.dev-local;JQNH45HN7V;XFBNJ82BV6`
+
+24. **Consumer Secret:** Paste the contents of the keyfile you've downloaded
+
+Save, and you now have Apple as a source.
+
+:::note
+For more details on how-to have the new source display on the Login Page see the Sources page.
+:::