What is the definitive source of rulings for the Pokemon TCG?
Official Pokémon ruling document regarding particular cards or their combinations is Pokémon TCG Rulings Compendium BW. It contains all rulings that were released by Pokémon Rules Team and approved by The Pokémon Company International.

If your card is not listed there, you can also look at similar mechanics, most of the time there is a card or a combination that fits your needs.

Your example of Decidueye-GX's Ability is covered by the TCG Rulebook though. It contains all the basic rules of the game.



  3 ATTACK AND END YOUR TURN
  On the first turn of the game, the starting player skips this step. (...)


  ABILITY: An Ability is an effect on a Pokémon that is not
  an attack. (...)



You can use an Ability on the first turn of the game if it doesn't say that you can't.