Update mkdocs (#180)

I think this is why there is no `index.html` and the site does not get
displayed?

Author: flora-hofmann-frequenz

mkdocs.yml

@@ -93,7 +93,7 @@ plugins:
   - literate-nav:
       nav_file: SUMMARY.md
   - mike:
-      alias_type: redirect
+      alias_type: symlink
       canonical_version: latest
   - mkdocstrings:
       default_handler: python