Refactor CLI/interface: init command, factory field, remove extras_size

- Rename `retro-gamer create` to `retro-gamer init` with positional
  args (GAME OUTPUT) instead of --game/--output flags
- Add [tool.retro-gamer].factory = "module:attr" to declare the game
  factory function from pyproject.toml instead of relying on a
  hard-coded create_game attribute
- Remove extras_size from user-facing config; it is now measured
  automatically from a sample observation and never declared
- Update docs throughout: create→init, runs/→training/, add factory
  field documentation, clarify [model] vs [training] hyperparameter
  sections, remove stale version-history explanations
- Bump version to 0.3.0; require retro-games>=2.5.0

docs/index.rst

@@ -58,7 +58,7 @@ Verify the installation by checking the command-line tool:
      Train and run RL agents for retro games.
 
    Commands:
-     create  Create a new training run directory with config.toml.
+     init    Initialize a new training run directory with config.toml.
      info    Print a summary of a training run.
      play    Watch a trained agent play the game.
      train   Train (or resume training) a DQN agent.