docs: add example showcasing engines

examples/4_engines_zoo.py

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+"""
+This example showcases all the different engines available and how you can switch between them.
+"""
+
+import os
+
+from kani import Kani, chat_in_terminal
+
+# fmt: off
+# ==== OpenAI (GPT) ====
+from kani.engines.openai import OpenAIEngine
+engine = OpenAIEngine(api_key=os.getenv("OPENAI_API_KEY"), model="gpt-3.5-turbo")
+
+# ==== Anthropic (Claude) ====
+from kani.engines.anthropic import AnthropicEngine
+engine = AnthropicEngine(api_key=os.getenv("ANTHROPIC_API_KEY"), model="claude-2.1")
+
+# ==== LLaMA v2 (Hugging Face) ====
+from kani.engines.huggingface.llama2 import LlamaEngine
+engine = LlamaEngine(model_id="meta-llama/Llama-2-7b-chat-hf", use_auth_token=True)  # log in with huggingface-cli
+
+# ==== LLaMA v2 (ctransformers) ====
+from kani.engines.ctransformers.llama2 import LlamaCTransformersEngine
+engine = LlamaCTransformersEngine(
+    model_id="TheBloke/Llama-2-7B-Chat-GGML",
+    model_file="llama-2-7b-chat.ggmlv3.q5_K_M.bin",
+)
+
+# ==== Vicuna v1.3 (Hugging Face) ====
+from kani.engines.huggingface.vicuna import VicunaEngine
+engine = VicunaEngine(model_id="lmsys/vicuna-7b-v1.3")
+
+# take your pick - the kani interface is compatible with all!
+ai = Kani(engine)
+
+if __name__ == "__main__":
+    chat_in_terminal(ai)
+
+# fmt: on