Prepare documentation site for pointing to fast.design instead of the default github pages URL

[janechu]

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

• I have included a change request file using $ npm run change
• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.