Rewrite the DAST browser-based documentation

Problem

It's unclear to users how to configure or troubleshoot DAST browser-based scans.

Proposal

• Review documentation to see what we have and what is missing
• Work out with PM/Technical Writer whether or not the DAST browser-based page should be stand-alone from the DAST ZAP page
• Work out if there should there be a separate page for how to migrate from a normal DAST ZAP scan to a DAST browser-based scan
• The top of the document should help the user get setup as soon as possible
• Document undocumented CI/CD variables
  • Look through the Browserker/DAST configuration file to make sure all relevant configuration settings have been exposed
  • Ensure we've documented settings according to this comment
• Document what "scope" means, and when it applies. It doesn't apply when authenticating (moved to #388900)
• Document mutual TLS (moved to #388900)
• Document undocumented log levels
• Document troubleshooting steps, for example, how to configure the log file
• Document how to configure DevTools logging
• Document how to configure Chromium logging
• won't do, not enough time Document what all the different timeouts mean
• Include a troubleshooting section
  • Document common errors, what they mean, and how to fix them
• Review authentication
  • Consider having it on a separate page, as it is used by both browser-based and ZAP scans
  • Document SSO support
  • Troubleshooting steps
  • Be clear about requirements for logging in, in particular, the random cookie/session ID value
• Document request headers, and how they are only sent to target host/allowed hosts (moved to #388900)
• Investigate #363195 (closed) for further ideas
• moved to new issue Add migration path from ZAP to browser-based scans
• wont do: we can get to this later How to update/pin the analyzer version
• Note somewhere that DAST does not validate SSL certificates (moved to #388900)
• Separate Available CI/CD variables into plumbing/porcelain
• Remove the DAST_BROWSER_EXCLUDED_HOSTS from available CI/CD variables
• For authentication, update the DAST_WEBSITE and configure logout URL
• Users can configure authentication cookies
• Authentication report shows empty request