Group mkdocs version bumps

There might be cases in which some mkdocs extensions pin certain minor
mkdocs versions. Then we're forced to update them in lockstep with the
main mkdocs version.

Signed-off-by: Juan-Luis de Sousa-Valadas Castaño <jvaladas@mirantis.com>

.github/dependabot.yml

@@ -25,3 +25,7 @@ updates:
     open-pull-requests-limit: 3
     schedule:
       interval: daily
+    groups:
+      mkdocs:
+        patterns:
+          - "mkdocs*"