docs: the fleet models, and changelog entry

changelog.d/1697-fleet-models.md

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+- Draw every 3D model from the project's procedural fleet set: 121 models
+  (about 2 MB, MIT like the code) chosen per vehicle by type and drawn life
+  size, replacing the eight third-party aircraft models. Flights, Military
+  Flights and Local ADS-B pick a model by ICAO type, class and emitter
+  category; Vessels, Transit and Rail now draw models too, under the same 3D
+  toggle and distances, a ship at its AIS length and beam and a tram, bus or
+  train in its mode or category colour. The whole set loads once when a layer
+  starts (samehkhamis, #1697).