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

Adds a design doc covering two related routing-contract extensions:

1. Multi-port routes: an optional Port override on RouteMatch so a
   single NebariApp can route different path prefixes to different
   ports on the same backend Service under one hostname.

2. Streaming timeouts: an opt-in routing.streaming flag that makes
   the operator emit an Envoy Gateway BackendTrafficPolicy disabling
   the default 15s request timeout (and setting a 5-minute idle
   timeout) for SSE, long-poll, and gRPC streaming workloads.

Also tightens the same-namespace contract by removing
ServiceReference.Namespace (nebari-landing's dependency on this field
is dormant and falls back gracefully) and codifies the "one NebariApp
= one hostname = one backend Service" boundary that has been implicit
until now.

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

Author: viniciusdc