Update documentation for manual installation of the new packages

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What does this MR do?

Update documentation for manual installation of the new packages

The way new gitlab-runner packages are released has been changed, with the runner binary decoupled from the helper images.

Why was this MR needed?

The documentation is out-of-date.

What's the best way to test this MR?

Using a Docker image and following the installation steps should be sufficient.

What are the relevant issue numbers?

This MR implements #38401 (closed).

Closes #38401 (closed)