docs: revise isKey/isButton checks and add examples

[imaginarny]

The most notable edition is making isButton..() methods obvious that param is optional, that check for any button is also possible, not only for certain. I've just stumbled across this in the docs and thought it's not possible.

As I was adding that, I went through all isButton..() and isKey..() check methods and added some basic examples as well, so there's use of all options like e.g.:

isKeyDown()
isKeyDown("jump")
isKeyDown(["left", "right"])

It covers the following:

isKeyDown()
isKeyPressed()
isKeyPressedRepeat()
isKeyReleased()

isButtonPressed()
isButtonDown()
isButtonReleased()

[pkg-pr-new]

Open in Stackblitz

pnpm add https://pkg.pr.new/kaplayjs/kaplay@484

commit: eaca214