Add Proxy-Wasm spec links and mention performance benefits

Add links to TCP stream and HTTP stream contexts in the Proxy-Wasm
spec, and clarify that operating at the TCP level is useful for
performance and protocol-agnostic routing decisions.

Signed-off-by: Christof Gerber <christof.gerber1@gmail.com>

Author: SiiiTschiii

examples/envoy_tcp_routing/README.md

@@ -2,7 +2,7 @@
 
 Proxy-Wasm TCP filter that dynamically routes connections to different upstream clusters based on the source IP address.
 
-This example operates at the TCP stream context (L4) rather than the HTTP application layer, making it useful for use cases where application-layer processing should be avoided while still enabling intelligent routing decisions.
+This example operates at the [TCP stream context](https://github.com/proxy-wasm/spec/tree/main/abi-versions/v0.2.1#tcp-streams) (L4) rather than the [HTTP application layer](https://github.com/proxy-wasm/spec/tree/main/abi-versions/v0.2.1#http-streams), making it useful for use cases where application-layer processing should be avoided for performance or protocol-agnostic routing decisions.
 
 This example is inspired by the [wasmerang](https://github.com/SiiiTschiii/wasmerang) project, which demonstrates advanced TCP routing patterns in Envoy/Istio/K8s using WASM filters.