Manual rpm and deb installation details

What does this MR do?

Provide details on how to manually install rpm and deb files.

Fix https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/1446

@bbodenmiller thanks for the MR. The instructions are certainly an improvement over what's there now, but I question why we document this installation method at all? Users installing like this won't get security updates, and can't verify package integrity either.

My preference would be to strongly recommend to users that they always set up the repositories correctly, and pretend that this option doesn't exist.

/cc @tmaczukin

In many enterprises your on-prem servers are not allowed to or able to connect to the public internet and as such you either need to a. run your own internal package manager (another pain), b. install packages like documented here, or c. install 100% manually which is less than fun and very error prone.

GitLab Omnibus documents this installation method and I think for consistency Runner should as well.

@bbodenmiller Should we instead download packages from s3, not from packagecloud?

https://gitlab-ci-multi-runner-downloads.s3.amazonaws.com/latest/index.html

Yep, if that is always up to date that would be easier. I'll update.

Updated to include details to discourage users from using this installation method as well as use the s3 links.

added Documentation label

resolved all discussions

What can I do to help get this merged?