Link to security suggestions on Install GitLab page

Why is this change being made?

Link to Securing your GitLab installation on about.gitlab.com install documentation in addition to docs.gitlab.com install docs

Helps with gitlab-org/gitlab#30968 (closed)

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

I'm honestly not sure who DRI of this page is. 😅

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what

Context

This MR (and it's sister MR) is the product of discussions in AppSec office hours and a chat with @ankelly about making an MVC that everyone can (hopefully) agree on.

Requested feedback and discussed this MR in AppSec Office Hours. There is ongoing discussion on other ways to increase awareness of this and/or use secure defaults, but seemed to be a consensus that this MVC is a step in the right direction.

@axil @mikelewis @jcolyer Ready for review! Any feedback/suggestions appreciated.