Update GitLab Security page with Security Best Practices

Doc link: https://docs.gitlab.com/ee/security/

Problem:

User Story: Self-Managed Customer's Security, Compliance, Sytems Admins want to know "how can I secure/harden my self-managed GitLab instance?"

Good Answers here:

• https://about.gitlab.com/blog/2020/05/20/gitlab-instance-security-best-practices/
• comments on #195380

Closes: #195380