Add SSH content to the Use Git section

What does this MR do and why?

This MR updates the SSH key information in the Install Git page.

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Comment from Suzanne:

I wonder if this might also go under Git? Hmmm. It's required, but it's not really about learning Git. Just putting here for consideration. Use SSH keys to communicate with GitLab.

Related issues

• Issue: #499459 (closed)
• Child item: #499465 (closed)
• MR to update navigation: Draft: Add SSH page to Use Git navigation (gitlab-docs!5151 - closed)