Prepare documentation site for pointing to fast.design instead of the…

… default github pages URL (#7014)

# Pull Request

## 📖 Description

This change:
- Resets the base URL to "/" as the site will be served from `https://ww.fast.design`
- Updates the URL to https://www.fast.design

## 👩‍💻 Reviewer Notes

These change must happen before updating the DNS and after publishing of the v2 FAST element package.

## ✅ Checklist

### General

<!--- Review the list and put an x in the boxes that apply. -->

- [ ] I have included a change request file using `$ npm run change`
- [ ] I have added tests for my changes.
- [x] I have tested my changes.
- [x] I have updated the project documentation to reflect my changes.
- [x] I have read the [CONTRIBUTING](https://github.com/microsoft/fast/blob/master/CONTRIBUTING.md) documentation and followed the [standards](https://github.com/microsoft/fast/blob/master/CODE_OF_CONDUCT.md#our-standards) for this project.

sites/website/docusaurus.config.js

@@ -3,8 +3,8 @@ import {themes as prismThemes} from 'prism-react-renderer';
 module.exports = {
     title: "FAST",
     tagline: "The adaptive interface system for modern web experiences",
-    url: "https://microsoft.github.io",
-    baseUrl: "/fast/",
+    url: "https://www.fast.design",
+    baseUrl: "/",
     deploymentBranch: "gh-pages",
     trailingSlash: false,
     favicon: "/favicon.ico",