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)

doc/topics/git/ssh_keys.md

@@ -7,3 +7,12 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 DETAILS:
 **Tier:** Free, Premium, Ultimate
 **Offering:** GitLab.com, Self-managed, GitLab Dedicated
+
+SSH keys provide secure communication between your computer and GitLab.
+When you use SSH keys to authenticate to the GitLab remote server,
+you don't need to supply your username and password each time.
+
+SSH uses two keys:
+
+- A public key that can be shared.
+- A private key that should be kept secure.