Guidelines for river and pingora frontend issues

It'd be nice to have clearer guidelines that indicate the division of responsibilities between [`river`](https://github.com/memorysafety/river) (a pingora "frontend" that compiles into an actual binary) and `pingora` (the core framework). We've been directing folks' issues to `river` when it makes sense.

Some classes of features such as no/low-code configuration are already set to belong [in `river` rather than `pingora`](https://github.com/memorysafety/river/blob/main/docs/what-to-build.md), but other cases may be less clear. For example, some features like Kubernetes ingress implementations per https://github.com/cloudflare/pingora/issues/41 are almost certainly destined for `river` and not core `pingora`, but the `river` implementation may generate a feature request for `pingora`; the exact division of code might not be clear until design time.

This is a tracking and discussion issue for what those clearer guidelines should be and where to describe them. The ideal outcome is that we minimize confusion and issue redirection between the two projects.