docs(xdposchain): port JSONRPC documentation from XDPoSChain

Add XDPoSChain API docs (admin/debug/eth/miner/net/rpc/txpool) with JSON response examples and update docs index navigation.

Author: gzliudan

docs/XDPoSChain/README.md

@@ -0,0 +1,15 @@
+# XDC blockchain JSONRPC API
+
+Modules:
+
+- [XDPoS](./XDPoS/XDPoS.md)
+- [admin](./admin/admin.md)
+- [debug](./debug/debug.md)
+- [eth](./eth/eth.md)
+- [miner](./miner/miner.md)
+- [net](./net/net.md)
+- [rpc](./rpc/rpc.md)
+- [txpool](./txpool/txpool.md)
+- [web3](./web3/web3.md)
+
+Notice: type `BlockNumber` is the block number in hexadecimal format or the string `latest`, `earliest`, `pending` or `finalized`.