fix(node-infrastructure): add to sidebar and restore Protocol Stack further reading link

Adds node-infrastructure after app-architecture in the explicit Concepts
sidebar (now that PR #209 has merged). Restores the Protocol Stack
further reading link that was blocked during cross-branch validation.

Author: marc0olo

docs/concepts/node-infrastructure.md

@@ -180,6 +180,7 @@ Because the integrity checker is part of the initramdisk, a malicious actor cann
 
 ## Further reading
 
-- [Glossary: replica](../references/glossary.md#replica): the software stack that runs inside GuestOS
+- [Protocol Stack](protocol/index.md): the four-layer architecture (peer-to-peer, consensus, message routing, execution) that runs inside GuestOS
+- [Glossary: replica](../references/glossary.md#replica): the replica process that implements the protocol stack
 
 <!-- Upstream: informed by Learn Hub articles "Overview" (Node Infrastructure), "Trusted Execution Environments" (migrated, source retired) -->

sidebar.mjs

@@ -80,6 +80,7 @@ export const sidebar = [
       { slug: "concepts/network-overview" },
       { slug: "concepts/canisters" },
       { slug: "concepts/app-architecture" },
+      { slug: "concepts/node-infrastructure" },
       {
         label: "Protocol Stack",
         collapsed: true,