Add links to GitLab SSH Fingerprints on SSH configuration page and in ssh.md

As discussed in https://gitlab.com/gitlab-com/infrastructure/issues/3889, users have a hard time finding the SSH fingerprints of the server when setting up their SSH connection for the first time. We need to make https://docs.gitlab.com/ee/user/gitlab_com/ more prominent.

Docs:

• https://docs.gitlab.com/ce/ssh/README.html
• https://docs.gitlab.com/ee/gitlab-basics/create-your-ssh-keys.html

GitLab.com

• Point to the docs in https://gitlab.com/profile/keys. That has to be a conditional and show only for GitLab.com. Ideally, the SSH host keys fingerprints should be exposed under every GitLab instance if the admin has enabled it. - This has been done in !93162 (merged)
• Mention https://gitlab.com/help/instance_configuration in https://gitlab.com/profile/keys