Single-encounter Pokemon are missing

[Hyreon]

In Generation IV, diamond pearl and platinum, many legendaries can be encountered exactly once. Some appear in static locations, while others will roam across the map once encountered. A third category exists for Spirtomb's bizarre encounter procedure.

Currently, the API does not include any encounter entries for any of these three methods. Dialga, Palkia and the (unreleased, event-only) Arceus have no data. This seems inconsistent with Generation III, which lists Mew's event-only encounter: https://pokeapi.co/api/v2/pokemon/151/encounters

My current project uses this API to calculate which games are required for which Pokemon. If this feature were not added, then I would have to manually add legendaries.

[SimplyBLGDev]

#738 adds these encounters data up to gen 4