Refactor the currently messed up SSH docs

http://docs.gitlab.com/ce/ssh/README.html provides some info on SSH keys, but not quite...

This page is linked from the profile/keys page where you upload your public key. Here are the next steps:

• Split ssh/README.md to user/profile/ssh_keys.md and user/project/deploy_keys.md and cross-link them.
• Refactor the profile/keys page to include some minimal documentation on how to create your SSH key, similar to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6217
• Deprecate http://docs.gitlab.com/ce/gitlab-basics/create-your-ssh-keys.html and move its info in user/profile/ssh_keys.md
• Refactor user/profile/ssh_keys.md
• Refactor user/project/deploy_keys.md
• Change links in views
• Create MR for EE

Part of docs restructure https://gitlab.com/gitlab-org/gitlab-ce/issues/3349

cc @ernstvn