Enable sast_configuration_ui feature flag by default

What does this MR do?

Enable sast_configuration_ui feature flag

This enables the SAST Configuration UI by default, and adds documentation for it.

Addresses #231357 (closed).

Screenshots

n/a

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done