docs: 更新 lighter-python-main 文档，添加示例代码和 API 说明，重构部分内容以提高可读性

docs/lighter/lighter-python-main/examples/transfer_update_leverage.py

@@ -0,0 +1,47 @@
+import asyncio
+import lighter
+
+
+from examples.secrets import BASE_URL, API_KEY_PRIVATE_KEY, ETH_PRIVATE_KEY
+
+API_KEY_INDEX = 10
+TO_ACCOUNT_INDEX = 9
+ACCOUNT_INDEX = 10  # replace with your account_index
+
+
+async def main():
+    client = lighter.SignerClient(
+        url=BASE_URL,
+        private_key=API_KEY_PRIVATE_KEY,
+        account_index=ACCOUNT_INDEX,
+        api_key_index=API_KEY_INDEX,
+    )
+    api_client = lighter.ApiClient(configuration=lighter.Configuration(host=BASE_URL))
+    info_api = lighter.InfoApi(api_client)
+    order_api = lighter.OrderApi(api_client)
+
+    auth_token, _ = client.create_auth_token_with_expiry()
+    fee_info = await info_api.transfer_fee_info(ACCOUNT_INDEX, authorization=auth_token, auth=auth_token, to_account_index=TO_ACCOUNT_INDEX) 
+    print(fee_info)
+
+    err = client.check_client()
+    if err is not None:
+        print(f"CheckClient error: {err}")
+        return
+    memo = "a"*32  # memo is a user message and it has to be exactly 32 bytes long
+    transfer_tx, response, err = await client.transfer(
+        ETH_PRIVATE_KEY,
+        usdc_amount=100,  # decimals are added by sdk
+        to_account_index=TO_ACCOUNT_INDEX,
+        fee=fee_info.transfer_fee_usdc,
+        memo=memo,
+    )
+    if err != None:
+       raise Exception(f"error transferring {err}")
+    print(transfer_tx, response)
+
+    lev_tx, response, err = await client.update_leverage(4, client.CROSS_MARGIN_MODE, 3)
+    print(lev_tx, response, err)
+
+if __name__ == "__main__":
+    asyncio.run(main())