Add robots.txt and sitemap

[Noah Tran]

Description

robots.txt and sitemap are essential for Google Bot. We should add it.

Acceptance Criteria

• Add robots.txt to the public folder.

# *
User-agent: *
Allow: /

# Host
Host: https://quick-diff.com

# Sitemaps
Sitemap: https://quick-diff.com/sitemap.xml

• Generate sitemap.xml file to the public folder: https://blog.logrocket.com/build-sitemap-generator-nextjs/