Add Draft.js logo to header

Summary:
**Summary**

Added the Draft.js logo to the header.

Also turn on on page navigation for docs - some of the headers will need to be
changed in some of the docs to get it all to work correctly.

**Test Plan**

Ran site locally
Closes #1768

Reviewed By: NoamELB

Differential Revision: D8148268

Pulled By: NoamELB

fbshipit-source-id: 2731fb31d87116f01de24e2953dcf3bcee0493c6

Author: JoelMarcey

.gitignore

@@ -5,3 +5,5 @@ build/
 node_modules
 website/src/docs/
 npm-debug.log
+!website/static/
+yarn-error.log

website/siteConfig.js

@@ -20,6 +20,7 @@ const siteConfig = {
       label: "GitHub"
     }
   ],
+  headerIcon: 'img/draftjs-logo.svg',
   /* colors for website */
   colors: {
     primaryColor: '#3B3738',
@@ -40,6 +41,9 @@ const siteConfig = {
 
   // Google analytics tracking id
   gaTrackingId: 'UA-44373548-19',
+
+  // Show page Table of Contents
+  onPageNav: 'separate',
 };
 
 module.exports = siteConfig;