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/README.md

@@ -0,0 +1,24 @@
+## Setup steps for testnet
+- Go to https://testnet.app.lighter.xyz/ and connect a wallet to receive $500
+- run `system_setup.py` with the correct ETH Private key configured
+  - set an API key index which is not 0, so you won't override the one used by [app.lighter.xyz](https://app.lighter.xyz/)
+  - this will require you to enter your Ethereum private key
+  - the eth private key will only be used in the Py SDK to sign a message
+  - the eth private key is not required in order to trade on the platform
+  - the eth private key is not passed to the binary
+  - copy the output of the script and post it into `create_cancel_order.py`
+  - the output should look like
+```
+BASE_URL = 'https://testnet.zklighter.elliot.ai'
+API_KEY_PRIVATE_KEY = '0xea5d2eca5be67eca056752eaf27b173518b8a5550117c09d2b58c7ea7d306cc4426f913ccf27ab19'
+ACCOUNT_INDEX = 595
+API_KEY_INDEX = 1
+```
+- start trading using
+  - `create_cancel_order.py` has an example which created an order on testnet & cancels it
+  - you'll need to set up both your account index, api key index & API Key private key
+
+## Setup steps for mainnet
+- deposit money on Lighter to create an account first
+- change the URL to `mainnet.zklighter.elliot.ai`
+- repeat setup step