feat(env): autoload .env/.env.local at import time; add example & docs

- Add `deepeval/env.py` and call `autoload_dotenv()` from `deepeval/__init__.py`
  so `.env.local` then `.env` are loaded automatically at import.
  Precedence: process env -> .env.local -> .env. Opt out with `DEEPEVAL_DISABLE_DOTENV=1`.
- Add `.env.example` with supported environment variables and provider keys
- Ignore `.env.local` in git.
- Update README and docs:
  - Getting Started: new “Configure Environment Variables” section.
  - Evaluation Flags: environment variables table and note on autoload behavior.
  - OpenAI integration: prefer `.env.local`, keep shell/CI and notebook alternatives.
  - Ollama integration: environment setup with `OLLAMA_HOST`.
- Add `python-dotenv` dependency.