docs: remove cloud providers docs from main index

Those docs are already referenced in the setup page in the toc tree so
we should just be able to keep them in the tree from the setup page
instead.

Signed-off-by: Arthur Outhenin-Chalandre <git@mrfreezeex.fr>

Author: MrFreezeex

Documentation/network/clustermesh/index.rst

@@ -20,6 +20,3 @@ Multi-cluster Networking
    load-balancing
    policy
    affinity
-   aks-clustermesh-prep
-   eks-clustermesh-prep
-   gke-clustermesh-prep