Convenience @Query initializers for Void DatabaseContext

[groue]

This pull request defines a few convenience @Query initializers that avoid specifying an environment key path when the Queryable request uses Void as a DatabaseContext (debugging or testing request).

For example, given this request:

struct PlayersRequest: Queryable {
    static var defaultValue: [Player] { [] }

    // PlayersRequest feeds from nothing (Void)
    func publisher(in _: Void) -> AnyPublisher<[Player], Error> {
        ...
    }
}

One can just write:

struct PlayerList: View {
    @Query(PlayersRequest()) // <- no explicit key path
    private var players: [Player]

    var body: some View {
        List(players) { player in ... }
    }
}

Also in this PR: moved long documentations into DocC extension files.

[groue]

Lol I just went against my own advice by easing the dependency on SwiftUI 😅 #40 (comment)

• remove the EnvironmentValues convenience!