Update capitalization in /administration, /api, /ci
and /customization

Code blocks should not be spaced 4 times, as this will
prevent the code from being colored. They should
also be spaced the same as the lists they are a part
of, to make reading easier.

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

After review, heading was changed and links to
it needed to be updated, and minor tweaks to
the issues docs such as note formatting

Configuration examples were moved into their own section for the purpose
of organizing and being able to link directly.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/23560#note_61966788

* fixed anchor for create MR from email
* added a screenshot for this feature

Add Security Concerns section to reply by email documentation

[ci skip]

See merge request gitlab/gitlabhq!2191

with 2-step verification.

fix #28131

[skip ci]

[ci skip]

Improve and finish the fallback to the In-Reply-To and References header for the reply-by-email feature

A few things to note:
- The IncomingEmail feature is now enabled even without a
  correctly-formatted sub-address
- Message-ID for new thread mail are kept the same so that subsequent
  notifications to this thread are grouped in the thread by the email
  service that receives the notification
  (i.e. In-Reply-To of the answer == Message-ID of the first thread message)
- To maximize our chance to be able to retrieve the reply key, we look
  for it in the In-Reply-To header and the References header
- The pattern for the fallback reply message id is "reply-[key]@[gitlab_host]"
- Improve docs thanks to Axil

[ci skip]

[ci skip]