Add security scanning to CI pipeline

What does this MR do and why?

Copies over security scanning from gitlab-docs, dropping Ruby-related scanners (Brakeman) and the deprecated nodejs scanner.

gitlab-docs version: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/.gitlab/ci/security.gitlab-ci.yml?ref_type=heads

Pipeline output

• Security report: https://gitlab.com/gitlab-org/technical-writing-group/gitlab-docs-hugo/-/pipelines/1257351018/security

Individual jobs:

• gemnasium: https://gitlab.com/gitlab-org/technical-writing-group/gitlab-docs-hugo/-/jobs/6651005045
• secret_detection: https://gitlab.com/gitlab-org/technical-writing-group/gitlab-docs-hugo/-/jobs/6651005051
• semgrep-analyzer: https://gitlab.com/gitlab-org/technical-writing-group/gitlab-docs-hugo/-/jobs/6651005048

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.