Enable sast_configuration_ui feature flag by default

This issue tracks the enablement of the sast_configuration_ui feature flag, and the addition of documentation for SAST Config UI.

The work to implement the SAST Configuration UI for %13.3 is being carried out under the sast_configuration_ui feature flag. Once the feature is implemented as described in that epic, the feature flag should be enabled, and the documentation added.

It will be removed completely by #235135 (closed).

Implementation plan

• Enable the feature flag by default in https://gitlab.com/gitlab-org/gitlab/blob/v13.2.0-rc44-ee/ee/app/controllers/projects/security/sast_configuration_controller.rb#L34
• Add documentation. See #216635 (comment 368549459).