Update repository_mirroring.md as per https://gitlab.com/gitlab-org/gitlab-ee/issues/10361#note_198445760

Delete trailing whitespace, fix blockquotes, fix note
boxes, with indentation, clean up tables, etc.

Correct the spacing of unordered markdown lists
in docs, to maintain standards of documentation,
in /user and others.

Previously, we used brackets to denote the tier badges,
but this made Kramdown, the docs site Markdown renderer,
show many warnings when building the site. This is now
fixed by using parentheses instead of square brackets.

This was caused by [PREMIUM] looking like a link to
Kramdown, which couldn't find a URL there.

See:
- https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/484
- https://gitlab.com/gitlab-org/gitlab-ce/issues/63800

Change assorted absolute links to ce and ee docs
to relative links

Signed-off-by: NJuergen Kosel <juergen.kosel@gmx.de>

Apply wording from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24834#note_138592349Signed-off-by: NJuergen Kosel <juergen.kosel@gmx.de>

Apply of the following suggestions:
- https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24834#note_137860004
- https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24834#note_137860005
- https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24834#note_137860006Signed-off-by: NJuergen Kosel <juergen.kosel@gmx.de>

As requested in issue https://gitlab.com/gitlab-org/gitlab-ce/issues/57062
describe how to set up a push mirror repository at GitLab.
Signed-off-by: NJuergen Kosel <juergen.kosel@gmx.de>

SSH Auth "Introduced" section had push/pull support versions labeled incorrectly.