Document code snippets baseDir

Summary: Explained how to use baseDir, and also made the site actually use proper code snippets, embedding the flipper site as an example.

Reviewed By: fabiomassimo

Differential Revision: D30572679

fbshipit-source-id: aae3dd07225eb9e147f8bf73529b02bfcb6be9d2

website/docusaurus.config.js

@@ -179,6 +179,11 @@ const siteConfig = {
         theme: {
           customCss: require.resolve('./static/css/custom.css'),
         },
+        staticDocsProject: 'flipper',
+        trackingFile: 'xplat/staticdocs/WATCHED_FILES',
+        'remark-code-snippets': {
+          baseDir: '..',
+        },
         enableEditor: true,
       },
     ],