Many code blocks are 4spaced, and they render in GitLab
without coloring as a result, even though they are
fenced with a language label. If in a list, other items
woll render as being in a code block too, even if not
meant to. This fixes all these issues, and cleans up
minor whitespace issues in /user, /security, /ssh
and /migrate_ci_to_ce docs.

Ensures we give instructions for how to create ssh keys
in a single place only.

Signed-off-by: NTakuya Noguchi <takninnovationresearch@gmail.com>

By default ssh-keygen uses an insecure password encryption method on the ssh key.
This change will add options to better protect the key.
See this page for more detail: https://latacora.singles/2018/08/03/the-default-openssh.html

Even if the user does not set a password, using the `-o` option needs to become
the new standard way to run `ssh-keygen`, since the default behavior is insecure.
Your help in establishing this option will be greatly appreciated and will help
improve overall security for your users and ssh users in general.

Originally submitted at
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6796.
Signed-off-by: NRémy Coutable <remy@rymai.me>

Signed-off-by: NRémy Coutable <remy@rymai.me>

[ci skip]

Users must not change the default filename or they will have to create a
suitable .ssh/config which is out-of-scope here.

The keytype field is actually `ssh-dss` for DSA keys and they will not
be stored as `id_rsa.pub`.

Note that newer version of ssh actually also support
`ecdsa-sha2-nistp256` and others so it is also misleading to assume the
field always starts with `ssh-`.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/875

Provide help for people who aren't SSH-clued by indicating
how to point to key files that have been named something
other than the default. A lot of people seem to assume that
the filename they choose is irrelevant and then wonder why
their private key is not being found by simple 'ssh' commands.