website/docs: update outdated custom CSS docs (#14441)

Signed-off-by: Jens Langhammer <jens@goauthentik.io>

website/docs/customize/interfaces/_global/customcss.mdx

@@ -1,57 +0,0 @@
-### Custom CSS
-
-To further modify the look of authentik, a custom CSS file can be created. Creating such a file is outside the scope of this document.
-
-import TabItem from "@theme/TabItem";
-import Tabs from "@theme/Tabs";
-
-<Tabs
-  defaultValue="docker-compose"
-  values={[
-    {label: 'docker-compose', value: 'docker-compose'},
-    {label: 'Kubernetes', value: 'kubernetes'},
-  ]}>
-  <TabItem value="docker-compose">
-Create a `docker-compose.override.yml` file and add this block to mount the custom CSS file:
-
-```yaml
-services:
-    server:
-        volumes:
-            - ./my-css-file.css:/web/dist/custom.css
-```
-
-Afterwards, run the upgrade commands from the latest release notes.
-
-  </TabItem>
-  <TabItem value="kubernetes">
-Create a ConfigMap with your css file:
-
-```yaml
-apiVersion: v1
-kind: ConfigMap
-metadata:
-    name: authentik-custom-css
-    namespace: authentik
-data:
-    custom.css: |
-        ...
-```
-
-Then, in the helm chart add this to your `values.yaml` file:
-
-```yaml
-volumes:
-    - name: custom-css
-      configMap:
-          name: authentik-custom-css
-volumeMounts:
-    - name: custom-css
-      mountPath: /web/dist/custom.css
-      subPath: custom.css
-```
-
-Afterwards, run the upgrade commands from the latest release notes.
-
-  </TabItem>
-</Tabs>

website/docs/customize/interfaces/_global/global.mdx

@@ -1,5 +1,3 @@
-## Global customization
+### Global customization
 
-import CustomCSS from "./customcss.mdx";
-
-<CustomCSS />
+See [Brand Settings](../../../sys-mgmt/brands.md#branding-settings)