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

pyproject.toml

@@ -1,11 +1,11 @@
 [project]
 name = "retro-gamer"
-version = "0.2.0"
+version = "0.3.0"
 description = "A toolkit for learning reinforcement learning by training agents to play retro games"
 readme = "README.md"
 requires-python = ">=3.11"
 dependencies = [
-    "retro-games>=2.3.1",
+    "retro-games>=2.5.0",
     "torch>=2.0",
     "numpy>=1.24",
     "click>=8.0",