Update docs about Security Configuration redesign

What does this MR do?

This updates the docs to include information about the Security Configuration redesign.

This also tweaks the appearance according to review suggestions:

• 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

Testing locally

1. Enable the security_configuration_redesign feature flag
2. Visit a non-Ultimate project's Security & Compliance -> Configuration page

TODO

• Get UX approval for enabling the feature for GitLab.com globally
• After this is merged, enable the flag globally for GitLab.com

Screenshots (strongly suggested)

These screenshots aren't directly related to this change, but simply reflect how the Security Configuration page will be displayed for non-Ultimate projects with the feature flag disabled/enabled.

Feature flag disabled	Feature flag enabled

On a large screen

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• [-] I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• [-] I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #331614 (closed)