Merge branch 'docs-add-threat-insights-metadata' into 'master'

Docs: Add metadata to Secure-related docs page

See merge request gitlab-org/gitlab!42039

doc/api/vulnerabilities.md

+---
+stage: Secure
+group: Threat Insights
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
+---
+
 # Vulnerabilities API **(ULTIMATE)**
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/10242) in [GitLab Ultimate](https://about.gitlab.com/pricing/) 12.6.