docs: document the policy for backporting documentation changes
docs: document the policy for backporting documentation changes

Documentation changes should be backported "as far as they go" on the
supported branches, so that users can get relevant information from the
documentation branch associated to the software version they run.
Document this as part as the criteria for backports.

Signed-off-by: Quentin Monnet <d6b8e48afb2534b213e391cab43016505747a234@isovalent.com>

michi-covalent/cilium,tklauser/cilium,cilium/cilium,michi-covalent/cilium,cilium/cilium,tklauser/cilium,tklauser/cilium,cilium/cilium,tklauser/cilium,michi-covalent/cilium,tklauser/cilium,cilium/cilium,cilium/cilium,michi-covalent/cilium,michi-covalent/cilium