feat(Alert): Update alert documentation
Purpose
Update the alert documentation to:
- Ensure parity with the latest design specs and GitLab UI examples.
- Add visuals to help differentiate alert variants and use in the page.
- Evaluate other related open issues (linked in #757 (closed)) to see if docs updated here can add clarity to, or even close them.
- Define behavior when an alert provides form error remediation.
Closes #757 (closed), #423 (closed)
Edited by Jeremy Elder