Update installation documentation with symlink step

In part of the documentation we are using gitlab-runner instead of gitlab-ci-multi-runner command. In the future we are planing to migrate to gitlab-runner command and drop gitlab-ci-multi-runner at all. Currently there is gitlab-runner -> gitlab-ci-multi-runner symlink created by default in DEB and RPM packages, but in manual installation methods we don't have such step.

We should review all installation documentation and make sure, that each method will create the gitlab-runner -> gitlab-ci-multi-runner symlink.

/cc @ayufan @markpundsack

Added ~60197 label

Yes! This is a great precursor step to renaming the actual binaries, packages, repos, etc.

Is there any problem in actually renaming the binary as part of the installation instructions? e.g. for Mac OX:

sudo curl --output /usr/local/bin/gitlab-runner https://gitlab-ci-multi-runner-downloads.s3.amazonaws.com/latest/binaries/gitlab-ci-multi-runner-darwin-amd64
sudo chmod +x /usr/local/bin/gitlab-runner

@markpundsack I like it :)

We can actually modify all our download instructions to output to gitlab-runner instead of gitlab-ci-multi-runner.

I kinda like it, but I also think it's kind of sneaky, and people might not notice the rename. Someone at some point is going to think they remember the steps, and they'll download without renaming, and then get confused. Making having an explicit symlink is safer. Even having an explicit rename is risky because of the re-download problem, which would result in two binaries of different versions.

Milestone changed to %Backlog

We should do that as part of rename of GitLab Runner

Milestone changed to %v1.7

Milestone changed to %v1.8

Milestone changed to %v1.9

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