Improve docs around `environment:action`

The following discussion from !36568 (merged) should be addressed:

• @dosuken123 started a discussion:

  Can you also update https://docs.gitlab.com/ee/ci/yaml/#environmentaction? The section still has a lot of old information that action is basically specified for stop action, but we should also mention prepare.

  Maybe we should put a table about the available keywords for environment:action:.

  key	description
  start	...
  stop	...
  prepare	...