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

retro_gamer/trainer.py

@@ -55,7 +55,6 @@ _INCOMPATIBLE_METADATA = {
     'character_set':        'the set of board characters (changes input layer size)',
     'board_size':           'the board dimensions (changes input layer size)',
     'observation_function': 'how the observation is computed (changes input representation)',
-    'extras_size':          'the size of the non-board portion of the observation (changes input layer size)',
 }
 _INCOMPATIBLE_PREPROCESSING = {
     'spatial':              'spatial vs non-spatial network type (changes network architecture)',