Update link to SSH page to avoid redirection

What does this MR do and why?

In: !83173 (merged) we moved:

• From: https://docs.gitlab.com/ee/ssh/index.html
• To: https://docs.gitlab.com/ee/user/ssh.html

The link in the UI is still valid for now, but this MR just gets ahead of our eventual removal.

CC @marcel.amirault @sarahgerman just FYI.

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