[docs] Enable Java syntax highlighting on website

Summary: Java is not enabled by default in Prism (docusaurus's syntax highlighter). Let's fix that.

Reviewed By: jvillard

Differential Revision: D22019432

fbshipit-source-id: cc624fd08

website/docusaurus.config.js

@@ -72,6 +72,9 @@ module.exports = {
       ],
       copyright: `Copyright © ${new Date().getFullYear()} Facebook, Inc. Built with Docusaurus.`,
     },
+    prism: {
+      additionalLanguages: ['java'],
+    },
   },
   presets: [
     [