Document Security Configuration page redesign

This change also tweaks the appearance of the redesigned Security
Configuration page:

- Adds static text to left column
- Limits the width of the page content
- Adjusts column fractions to 4:8
- Fixes a bug where the configuration history link wasn't displaying
- Moves a line-height class to the SectionLayout component
- Changes the description of DAST Scans to better reflect the feature

Part of https://gitlab.com/gitlab-org/gitlab/-/issues/331614.