docs: include cluster-management.md in TOC (argoproj#19964)

While it is searchable in the docs UI, the Cluster Management page is
not included in the Table of contents of https://argo-cd.readthedocs.io

Add it to increase it's discoverability.
We put it just before cluster-bootstrapping since an user would be
expected to add a cluster to argocd just before "bootstrapping"

Signed-off-by: Max Gautier <mg@max.gautier.name>

mkdocs.yml

@@ -47,6 +47,7 @@ nav:
     - snyk/index.md
     - operator-manual/signed-release-assets.md
   - operator-manual/tls.md
+  - operator-manual/cluster-management.md
   - operator-manual/cluster-bootstrapping.md
   - operator-manual/secret-management.md
   - operator-manual/disaster_recovery.md