Allow configuring what version is shown by default (#554)

* [RFC] Allow configure what version is shown by default

Seeing if this will work - it won't work if you want to show `master` or `next` by default.
Only named version that would exist in `versioned_docs`.

If this does work and we want to go with it - `latestVersion` in many places seems like a
misnomer. `defaultVersion` would be better or something

* Remove from docusaurus siteconfig

lib/server/env.js

@@ -8,6 +8,7 @@
 const CWD = process.cwd();
 const fs = require('fs-extra');
 const path = require('path');
+const siteConfig = require(CWD + '/siteConfig.js');
 
 const join = path.join;
 
@@ -53,7 +54,9 @@ class Versioning {
     if (fs.existsSync(versions_json)) {
       this.enabled = true;
       this.versions = JSON.parse(fs.readFileSync(versions_json, 'utf8'));
-      this.latestVersion = this.versions[0];
+      this.latestVersion = siteConfig.defaultVersionShown
+        ? siteConfig.defaultVersionShown
+        : this.versions[0]; // otherwise show the latest version (other than next/master)
     }
   }
 }