geth: add page (#1917)

* initial commit - added tldr for geth

* fixed build errors

pages/common/geth.md

@@ -0,0 +1,19 @@
+# geth
+
+> The go-ethereum command line interface.
+
+- Connect to the main Ethereum network and automatically download the full node:
+
+`geth`
+
+- Connect to the Ropsten test network:
+
+`geth --testnet`
+
+- Create a new account:
+
+`geth account new`
+
+- Enable mining:
+
+`geth --mine`