docs(changelog): add the #1556 fragment

changelog.d/1556-cockpit-own-layer-entry.md

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+- Enter Cockpit with only the ridden aircraft's layer on. Contacts is no longer
+  required; turning off the aircraft's own feed ends the view, a missing other
+  feed reads `… FEED OFF · PARTIAL PICTURE`, and double-clicking the followed
+  aircraft or the entry beside its readout enters it (Denis Groshev, @ZloyS,
+  #1556).