[BUG][online docs] Broken links in top-level README.md

Created by: zchamski

Is there an existing CVA6 bug for this?

• I have searched the existing bug issues

Bug Description

The top-level README.md contains broken links:

• Build Status results in a "Content length exceeded" error
• CVA6 dashboard points to wrong location ("Prerequisites" section of the README file instead of the actual dashboard).