Improve SSH documentation page to reflect gitlab-sshd changes

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Docs: https://docs.gitlab.com/ee/user/ssh.html

When gitlab-com/gl-infra&603 (closed) is completed and gitlab-sshd is release and stable, we need to improve the SSH documentation page in order to reflect all the changes related to gitlab-sshd.

For example, the support for DSA SSH keys is removed: https://docs.gitlab.com/ee/user/ssh.html#see-if-you-have-an-existing-ssh-key-pair.

It would be useful to have the tables for the MACs, Ciphers and Key exchanges algorithms that we currently support: gitlab-shell#575 (closed)