use splash gradient in footer, switch to GitHub icon (#1998)

Summary:
This small PR aims to tweak a bit website appearance and fix small UI issue:
* footer background has been changed to reversed splash gradient
  <img width="1104" alt="Screenshot 2021-03-04 131823" src="https://user-images.githubusercontent.com/719641/109964907-8cba1c80-7cee-11eb-8905-2fa82533cfd7.png">
* GitHub link in the Navbar has been moved to the tight and text was replaced with icon (like on few other Docusaurus sites)
  <img width="557" alt="Screenshot 2021-03-04 133112" src="https://user-images.githubusercontent.com/719641/109965233-f9351b80-7cee-11eb-84d6-30788f662fbf.png">
* Navbar brand title ("Flipper") hover color issues has been fixed, now hover effect is the same as other navbar links

## Changelog

N/A

Pull Request resolved: https://github.com/facebook/flipper/pull/1998

Test Plan: Flipper website run on `localhost`.

Reviewed By: mweststrate

Differential Revision: D26888499

Pulled By: passy

fbshipit-source-id: e8abeace8c1d6ae8dcc46c2381009c2bd2500005

website/docusaurus.config.js

@@ -51,16 +51,17 @@ const siteConfig = {
           label: 'Creating Plugins',
           position: 'right',
         },
-        {
-          href: repoUrl,
-          label: 'GitHub',
-          position: 'right',
-        },
         {
           to: 'docs/internals/index',
           label: 'Under the Hood',
           position: 'right',
         },
+        {
+          href: repoUrl,
+          position: 'right',
+          'aria-label': 'GitHub repository',
+          className: 'navbar-github-link',
+        },
       ],
     },
     colorMode: {