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

The docker executor does not work the same as the others. Add a note for
the users and points them to an issue that describes the issue.

closes gitlab-org/gitlab-ce#51295

Create a new page for the interactive web terminals since it doesn't fit in any
other place.

Add link in CI home page.