Switch to mkdocs-materialx

The project mkdocs-material is in maintenance-only mode and their
successor is not ready yet.

Use the modern, refreshed theme and drop the pymdownx.magiclink
extension.

mkdocs.yml

@@ -15,7 +15,7 @@ copyright: Copyright © 2026 Headscale authors
 
 # Configuration
 theme:
-  name: material
+  name: materialx
   features:
     - announce.dismiss
     - content.action.edit
@@ -138,10 +138,6 @@ markdown_extensions:
       pygments_lang_class: true
   - pymdownx.inlinehilite
   - pymdownx.keys
-  - pymdownx.magiclink:
-      repo_url_shorthand: true
-      user: squidfunk
-      repo: mkdocs-material
   - pymdownx.mark
   - pymdownx.smartsymbols
   - pymdownx.superfences: