docs: add rpc quick tests examples

Adds RPC_QUICKTESTS.md with a few JSON-RPC curl examples to quickly validate node health.

docs/RPC_QUICKTESTS.md

@@ -0,0 +1,33 @@
+# RPC Quick Tests
+
+This document lists a few simple JSON-RPC calls that can be used to
+verify that a geth node is responding correctly.
+
+All examples use `curl` against a local HTTP endpoint:
+
+    http://127.0.0.1:8545
+
+## Get the current block number
+
+    curl -X POST http://127.0.0.1:8545 \
+      -H "Content-Type: application/json" \
+      -d '{"jsonrpc":"2.0","method":"eth_blockNumber","params":[],"id":1}'
+
+## Get the node's client version
+
+    curl -X POST http://127.0.0.1:8545 \
+      -H "Content-Type: application/json" \
+      -d '{"jsonrpc":"2.0","method":"web3_clientVersion","params":[],"id":1}'
+
+## Get the peer count
+
+    curl -X POST http://127.0.0.1:8545 \
+      -H "Content-Type: application/json" \
+      -d '{"jsonrpc":"2.0","method":"net_peerCount","params":[],"id":1}'
+
+If these calls return valid JSON responses, the node is up and the
+HTTP JSON-RPC endpoint is reachable. If they fail, check:
+
+- that `--http` is enabled,
+- that the correct `--http.addr` and `--http.port` are in use,
+- any firewall rules or container port mappings.