docs: materialx is deprecated, switch to material.extensions

Build failed with the following warning:
```
DeprecationWarning: 'materialx.emoji.twemoji' is deprecated.
Material emoji logic has been officially moved into mkdocs-material
version 9.4. Please use Material's 'material.extensions.emoji.twemoji'
as mkdocs_material_extensions is deprecated and will no longer be
supported moving forward. This is the last release.
```

Signed-off-by: Timo Beckers <timo@isovalent.com>

docs/mkdocs.yml

@@ -148,8 +148,8 @@ markdown_extensions:
       alternate_style: true 
   # Emoji and icons like :fontawesome-brands-golang:
   - pymdownx.emoji:
-      emoji_index: !!python/name:materialx.emoji.twemoji
-      emoji_generator: !!python/name:materialx.emoji.to_svg
+      emoji_index: !!python/name:material.extensions.emoji.twemoji
+      emoji_generator: !!python/name:material.extensions.emoji.to_svg
       options:
         custom_icons:
           - overrides/.icons