Add config option for showing story page on game start

Make a togglable/boolean config option for showing the story page when the game starts. Right now it always shows it and players don't get a choice.

Locate the game-start entry point where the story page is currently always shown, along with the existing configuration definitions. Add a boolean option whose enabled and disabled settings determine whether the story page appears at startup, and verify both settings in OpenTTD.

Written by the indexing model from the issue text.