chore: let docs approvers approve mkdocs.yml (argoproj#16134)

@argocd-approvers-docs cannot approve argoproj#15689 since `mkdocs.yml` is not
included in the `CODEOWNERS` file. This should be pretty benign to add.

Signed-off-by: Blake Pettersson <blake.pettersson@gmail.com>
Signed-off-by: Kevin Lyda <kevin@lyda.ie>

CODEOWNERS

@@ -2,8 +2,9 @@
 ** @argoproj/argocd-approvers
 
 # Docs
-/docs/**  @argoproj/argocd-approvers @argoproj/argocd-approvers-docs
-/USERS.md @argoproj/argocd-approvers @argoproj/argocd-approvers-docs
+/docs/**    @argoproj/argocd-approvers @argoproj/argocd-approvers-docs
+/USERS.md   @argoproj/argocd-approvers @argoproj/argocd-approvers-docs
+/mkdocs.yml @argoproj/argocd-approvers @argoproj/argocd-approvers-docs
 
 # CI
 /.github/**       @argoproj/argocd-approvers @argoproj/argocd-approvers-ci