docs(design): propose multi-port routes for NebariApp

Adds a design doc proposing an optional per-route port override on
RouteMatch so a single NebariApp can route different path prefixes to
different ports on the same backend Service under one hostname. Tightens
the same-namespace contract by removing ServiceReference.Namespace, and
codifies the "one NebariApp = one hostname = one backend Service"
boundary that has been implicit until now.

The streaming/BackendTrafficPolicy concern (Envoy SSE timeouts) is
referenced as a follow-up but is intentionally not part of this proposal.

Filename kept as multi-backend-routes.md for URL stability; the doc
explains the rename.

Author: viniciusdc