Add upgrade troubleshooting info to Runner docs

What does this MR do?

As per this issue discussion, adds troubleshooting info about when gitlab-runner 15.7.x and 15.8.x doesn't start if it lacks write permission for the directory containing the config.toml file.

gitlab-runner 15.7.x and 15.8.x may refuse to start if it lacks write permission for the directory containing the config.toml file. It looks for a .runner_system_id file in that directory, and if it doesn't find a valid one, it will try to create a new one. In version 15.9, the issue was resolved by providing guidance to the user on how to manually create the file if necessary. Alternatively, users can temporarily allow file write permissions by running the gitlab-runner run command once. Once the .runner_system_id file is created, the permissions can be set back to read-only.

Since this error can occur in upgrades in multiple platforms (K8s, Docker, Windows), for now I'm adding this troubleshooting info to the GitLab Runner commands page so that the installation docs for each platform can link here. This seems like currently the best location for this info.

Related issues

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