For the sake of consistency, removes any extraneous
consecutive blank lines from the doc suite.

`queue_duration` is a useful metric that is currently in api_json.log
but not in production_json.log. We should add it because it tells us how
long the request sat in Workhorse before Unicorn processed it. Having
this field enables the support team to better troubleshoot when delays
began to happen.

Instead of requiring one separate document for every version (which is
created by release managers), we now use a single document for both
Community Edition and Enterprise Edition. This allows developers to add
guidelines right away, instead of release managers having to determine
what to add.

Version specific upgrading instructions are placed in a separate
section. This prevents instructions from lingering around for many
versions should a developer forget to remove them. This also ensures
that all instructions are kept in a single place, instead of being
spread across different documents.

A minor downside is that CE and EE guidelines now live in the same
document, which could cause merge conflicts. Since we are working
towards a single codebase, this should only be an issue until we merge
the codebases together; something we expect to do in the coming months.

This commit also removes all old upgrading instructions, but these can
still be accessed using Git branch specific URLs (included in the
updating README).

Cached markdown version is composed both from global and local
markdown version. This allows admins to bump version locally when
needed (e.g. when external URL is changed).

Update the list of GitLab metrics to include the missing metric names.

- Also makes other minor Markdown fixes that were near the main fixes.

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

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

This reverts commit cc735352.

I pushed this commit to master accidentally. Oops!