Replace JSON config with .env files

- Add python-dotenv dependency for environment variable loading
- Replace params.json and params-local.json with .env files
- Add --local and --local-stacosys flags to makesite.py
- Update Makefile to auto-detect stacosys availability
- Document configuration and usage in README.md

pyproject.toml

@@ -10,6 +10,7 @@ requires-python = ">=3.12.9"
 dependencies = [
     "mistune>=3.0.2",
     "pygments>=2.18.0",
+    "python-dotenv>=1.0.0",
     "requests>=2.32.3",
 ]