doc/rados: document require_min_compat_client

doc/rados: document require_min_compat_client

Add a standalone concept page for the OSDMap require_min_compat_client
field, covering: how to set and check it, the non-monotonic lowering
behavior (with the features-in-use floor derived from
OSDMap::get_min_compat_client()), and the operator commands it gates.
Include tables for the floor-pinning features and the flag-gated
commands, so operators can reason about transitions without reading
OSDMonitor.cc.

Cross-reference to the CephFS per-filesystem required_client_features
mechanism, which is the MDSMap-side equivalent for client-protocol
features. Add an anchor on the existing CephFS Required Client Features
section so the cross-reference resolves.

Link the new page from the rados operations index.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>