Refactor SSH key addition in GitLab basics

doc/gitlab-basics/create-your-ssh-keys.md

doc/gitlab-basics/img/add_sshkey.png

doc/gitlab-basics/img/key.png

doc/gitlab-basics/img/paste_sshkey.png

doc/gitlab-basics/img/profile_settings.png

doc/gitlab-basics/img/profile_settings_ssh_keys.png

...gitlab-basics/img/profile_settings_ssh_keys_paste_pub.png

...itlab-basics/img/profile_settings_ssh_keys_single_key.png

doc/gitlab-basics/img/profile_settings_ssh_keys_title.png

doc/gitlab-basics/img/shh_keys.png