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

Makefile

@@ -4,7 +4,7 @@ CF_DISTRIBUTION = EPA6NHZ2LEH1A
 .PHONY: build deploy clean
 
 build:
-	uv run --group documentation $(MAKE) -C docs html
+	uv run --group documentation sphinx-build -M html docs docs/_build
 
 deploy: build
 	aws s3 sync docs/_build/html $(S3_BUCKET)