add Google Analytics for documentation pages (#2645)

We need Google Analytics to start measuring how many developers are viewing our documentation. That way we can gauge how successful an/or useful various pages are. VuePress supports GA and all we have to provide is the tracking code.

This PR also renames the static docs site to "Tendermint Documentation", which is a better representation of the contents than only "Tendermint Core".

docs/.vuepress/config.js

@@ -1,6 +1,7 @@
 module.exports = {
-  title: "Tendermint Core",
-  description: "Documentation for Tendermint Core",
+  title: "Tendermint Documentation",
+  description: "Documentation for Tendermint Core.",
+  ga: "UA-51029217-1",
   dest: "./dist/docs",
   base: "/docs/",
   markdown: {