Remove unneeded trailing whitespace in lines in
/development /ci /api and /administration docs.

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

For ssot epic

The current documentation suggests that a user can not use his own
runner for a specific group or project. At first I thought this was the
case but it is not true, since we do allow websocket connections on
GitLab.com, this was discovered as part of the investigation on
https://gitlab.com/gitlab-org/gitlab-ce/issues/52611.

In the administration page for web terminals add a new `Security`
section explaining how the interactive web terminals are secured between
GitLab and gitlab-runner.

This section is under administration instead of
`doc/ci/interactive_web_terminal` because the administrators of the
GitLab instance would be mostly interested in this, whilst
`doc/ci/interactive_web_terminal` are interested in how the feature
works.

closes https://gitlab.com/gitlab-org/gitlab-ce/issues/52681

We have had multiple issues on why web terminals are not working on
GitLab.com, there is current work in progress to make it stable and
work in https://gitlab.com/gitlab-org/gitlab-ce/issues/52611

Add note in docs for docker executor for interactive web terminal

See merge request gitlab-org/gitlab-ce!21880