docs(guide): add section for Authentik integration (#34365)

- Split authentication section of the guide into two sub-sections:
Authelia and Authentik.
- Add detailed instructions for configuring Authentik for domain-level
forward auth.

**Description**
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📘 Added guide section for configuring Authentik with NGINX
Ingress Controller. This was motivated by a thread in the Discord server
where I shared my configuration with another user who was asking
questions and @PrivatePuffin suggested I contribute to the docs 😃

**⚙️ Type of change**

- [ ] ⚙️ Feature/App addition
- [ ] 🪛 Bugfix
- [ ] ⚠️ Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] 🔃 Refactor of current code
- [x] 📜 Documentation Changes

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areas
- [x] 📄 I have made changes to the documentation
- [ ] 🧪 I have added tests to this description that prove my fix is
effective or that my feature works
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- [x] I made sure the title starts with `feat(chart-name):`,
`fix(chart-name):`, `chore(chart-name):`, `docs(chart-name):` or
`fix(docs):`

**➕ App addition**

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---

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---------

Signed-off-by: Alfred Göppel <43101280+alfi0812@users.noreply.github.com>
Co-authored-by: Alfred Göppel <43101280+alfi0812@users.noreply.github.com>

website/src/content/docs/guides/ingress/nginx.md

@@ -137,7 +137,7 @@ annotations:
 
 ### Auth
 
-For Authelia, Authentik and more
+#### Authelia
 
 ```yaml
 annotations:
@@ -147,6 +147,30 @@ annotations:
   nginx.ingress.kubernetes.io/auth-response-headers: 'Remote-User,Remote-Name,Remote-Groups,Remote-Email'
 ```
 
+#### Authentik
+
+When using Authentik, take care to configure the service as follows. 
+
+```yaml
+annotations:
+  nginx.ingress.kubernetes.io/backend-protocol: "HTTPS"
+  nginx.ingress.kubernetes.io/ssl-passthrough: "true"
+  nginx.ingress.kubernetes.io/force-ssl-redirect: "true"
+```
+
+For domain-level forward auth, you must configure the embedded outpost first (please refer to
+[Authentik's docs](https://truecharts.org/charts/stable/authentik/how_to/)). The basic steps are to create a provider and application, then enable the embedded outpost for your newly created application.
+
+Once that has been done, configure each service you wish to place behind Authentik as follows:
+
+```yaml
+annotations:
+  nginx.ingress.kubernetes.io/auth-url: http://authentik-http.authentik.svc.cluster.local:10230/outpost.goauthentik.io/auth/nginx
+  nginx.ingress.kubernetes.io/auth-signin: https://auth.${DOMAIN_1}/outpost.goauthentik.io/start?rd=$scheme://$http_host$escaped_request_uri
+  nginx.ingress.kubernetes.io/auth-response-headers: Set-Cookie,X-authentik-username,X-authentik-groups,X-authentik-entitlements,X-authentik-email,X-authentik-name,X-authentik-uid
+  nginx.ingress.kubernetes.io/auth-snippet: proxy_set_header X-Forwarded-Host $http_host;
+```
+
 ### IP Whitelist
 
 ```yaml