[docs] Add hash group identity KEP (#986)

* Add KEP-898: Hash group identity

Signed-off-by: Benjamin Braun <benjaminbraun@google.com>

* Add DisaggregatedSet integration to KEP-898

Signed-off-by: Benjamin Braun <benjaminbraun@google.com>

* State that the Ordinal default is permanent in KEP-898

Signed-off-by: Benjamin Braun <benjaminbraun@google.com>

* Add downgrade risk and group key rationale to KEP-898

Signed-off-by: Benjamin Braun <benjaminbraun@google.com>

* Keep KEP-898 implementation history to milestones only

Signed-off-by: Benjamin Braun <benjaminbraun@google.com>

* Document NoneRestartPolicy semantics in hash mode

Signed-off-by: Benjamin Braun <benjaminbraun@google.com>

* Tighten KEP-898 wording

Signed-off-by: Benjamin Braun <benjaminbraun@google.com>

* KEP: leaders get group key derived DNS names, support subGroupPolicy and UniquePerReplica in hash mode

Signed-off-by: Benjamin Braun <benjaminbraun@google.com>

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Signed-off-by: Benjamin Braun <benjaminbraun@google.com>