Draft: POC app routing

What does this MR do and why?

Related issue: #428195 (closed)

Sets up minimal code structure in order for Routing layer to be able to route to correct cell.

Use https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/howto/cells.md#install-another-gdk-to-act-as-a-cell to try this out

Only certain routes are routable

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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