docs: document install_proxy flag for proxy kernel visibility

Since 26.24.0, a service's IPython kernel is only exposed as a proxy kernel in the JupyterLab interface if install_proxy = true is set on its entry in orchestration.toml. This wasn't documented, leading users to wonder why their services stopped appearing after upgrading.

Adds a short explanation and example snippet to the Through a proxy kernel section of the Accessing Services docs.