Add install instructions for the step-runner

Cameron Swordsrequested to merge
cam/docs-install-step-runner into main

What does this MR do?

Adds documentation for installing the step-runner binary, which enables GitLab Functions on executors without native support.

Why was this MR needed?

Users need guidance on when and how to install step-runner manually for their executor type.

What's the best way to test this MR?

Review the rendered documentation at docs/install/step-runner.md.

What are the relevant issue numbers?

N/A

Edited by Cameron Swords

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