Adding documentation for security hardening

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR adds detailed documentation on hardening a GitLab instance. There is one main document that references the others. The overall documentation has been broken out into specific pages that cover one area or topic each.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

As this is documentation only there is no need for numbered steps.

MR 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 MR.