Rename customize rulesets nav entry

Add navigation item to global navigation

• Link to the merge request that introduced the new page: gitlab!160338 (merged)
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page: https://docs.gitlab.com/ee/user/application_security/secret_detection/pipeline/custom_rulesets_schema.html

Note: In gitlab!160338 (merged) this page's file was renamed, hence the need to update the navigation entry.