update API doc with instructions

Author: kmurphypolygon

docs/zkEVM/api/json-rpc.md

@@ -3,4 +3,34 @@ hide:
 - toc
 ---
 
+## Terminal
+
+Use the Cardona testnet https://rpc.cardona.zkevm-rpc.com to test the [endpoint methods](#playground) in a terminal. For example: 
+
+### `zkevm_estimateGasPrice`
+
+```sh
+curl -X POST \
+  -H "Content-Type: application/json" \
+  --data '{"jsonrpc":"2.0","method":"zkevm_estimateGasPrice","params":[{"from":"0x0000000000000000000000000000000000000001","to":"0x0000000000000000000000000000000000000002","value":"0x1"}],"id":1}' \
+    https://rpc.cardona.zkevm-rpc.com
+```
+
+#### Result
+
+```sh
+{"jsonrpc":"2.0","id":1,"result":"0xe2ea2f0"}
+```
+
+## Playground
+
+The available methods are detailed in the playground description below.
+
+Each method description provides:
+
+- Method name and explanation.
+- Parameters required if any and their details. 
+- Expected return.
+- Examples.
+
 <embed type="text/html" src="https://playground.open-rpc.org/?schemaUrl=https://raw.githubusercontent.com/0xPolygon/polygon-docs/3eb44779e7380e91e5c92f160424159a3da1bdba/docs/zkEVM/api/zkevm.openrpc.json&uiSchema[appBar][ui:input]=false&uiSchema[appBar][ui:splitView]=false" width="100%" height="1000px">