[Docs] Card implementation guide

[rgraciano]

It would be helpful if we had a card implementation guide to go wider with card development. From my notes:

• cardApi: anything that manipulates the game board, could trigger other effects, etc goes here
• giveBonus/removeBonus: any kind of conditional bonuses given to other cards go here
• Abilities: anything that needs to be chosen on the front end, targets other cards, etc goes here. 3 types (normal, staging, handler).
• Document the difference between an alteration, a hook, and a handler, and when to use each

[rgraciano]

Some examples for how cards could be implemented:

Murkwood Allies:

• Boost, Beast, and Frogs are options
• Then spell should be discarded

Behind the Ferns

• Implements onLeaves, checks for Feral hero leaving
• When played, has only one (or no) abilities so user choice clears immediately
  Implements giveBonus
• Has to stay in Play (Channeling)

Moment’s Peace

• Add card to Effects list and to discard; having two copies of the card should be fine
  Implements a couple of hooks not yet created that get checked on getAllActiveCards stuff
• On start of turn, needs a property that tells game to remove from active Effects (forget card)

Stalking Tiger

• Needs giveBonus/removeBonus from Feral hero for the invisible status
  Implements StealthWhenAttacking
• Test: detected by tower after choosing a target