Rename FB Internal to Under the Hood

Summary:
I think there's a need for an "implementation details" section of the site. Especially as we're moving the rest of our wiki content there.

I see it as a good thing, because now we have a place for implementation details relevant to OSS contributors too, it's not all fb-specific.

Reviewed By: nikoant

Differential Revision: D25614909

fbshipit-source-id: 5ec081e6450a9ad1c30785ae4cca74b6fa0d2630

website/docusaurus.config.js

@@ -56,15 +56,11 @@ const siteConfig = {
           label: 'GitHub',
           position: 'right',
         },
-        // start-internal-navbar-example
-        ...fbInternalOnly([
-          {
-            to: 'docs/fb/index',
-            label: 'FB Internal',
-            position: 'right',
-          },
-        ]),
-        // end-internal-navbar-example
+        {
+          to: 'docs/internals/index',
+          label: 'Under the Hood',
+          position: 'right',
+        },
       ],
     },
     colorMode: {