Write documentation for the Frontend of First class Vulnerabilities

Problem to solve

We've been working away adding features for the First class vulnerabilities but don't have (frontend) docs for them yet. We'll need to add in these docs to describe the current state of the first class vulnerabilities.

Further details

The current plan is turn on the feature flag by default for all customers in 12.10 #204847 (comment 324378058).

TODOs:

• Update screenshots:
  • https://docs.gitlab.com/ee/user/application_security/security_dashboard/index.html
  • https://docs.gitlab.com/ee/user/application_security/index.html#interacting-with-the-vulnerabilities
• Add an ## Export vulnerabilities section in https://docs.gitlab.com/ee/user/application_security/security_dashboard/index.html and basically describe this video https://www.youtube.com/watch?v=e8mHU1EERkw !29485 (merged)