Remove Sourcegraph Cloud option

Created by: zlonko

This closes #5508 (closed). It removes the option for Sourcegraph Cloud from "Get Started" and adds the appropriate redirects and links.

6/23/22: Requirements not finalized

Changelog

• Updates "Get Started" CTA on homepage and nav /get-started/self-hosted
• Redirects /get-started to /get-started/self-hosted
• Removes 'Deployment options' on /get-started/self-hosted
• Updates the SelfHostedSection.tsx component's CTA copy to "Learn more about self-hosted" and link to /get-started/self-hosted

Test

1. Ensure prettier has standardized the proposed changes.
2. Please test links in nav and homepage
3. Please test redirect when visiting /get-started