docs: add How Routing Peers Work concept page (#727)

* docs: add How Routing Peers Work concept page

Add a feature-agnostic primer covering the routing peer mental model:
traffic flow through forward vs input chains, host requirements (IP
forwarding on Linux/Windows, container caps, cloud settings), HA modes
(primary/failover and equal-metric latency switching), masquerade,
access control, DNS routing, exit nodes, and common pitfalls. Surface
it in the sidebar under Networks and cross-link from both the Networks
and Network Routes concept pages.

* docs(routing-peers): apply review feedback

- Clarify mental-model walkthrough describes the Linux kernel-mode path
- Expand OS list to Linux/Windows/macOS/FreeBSD/Android/tvOS/Docker
- Correct container IP-forwarding guidance: set sysctl on the host
- Reframe Windows NB_ENABLE_LOCAL_FORWARDING as opt-in for exposing
  services on the routing peer's local addresses, off by default
- Soften failover wording to avoid implying tunnel rebuild; scope
  reset claim to established TCP connections, drop "typically"
- Add masquerade-off caveats: Linux only, breaks HA
- Drop unfounded ACL-Groups-require-masquerade Warning
- Drop misplaced internal-DNS-resolver link from DNS section intro
- Rename Wildcard domains section to Routing Peer DNS Resolution
- Drop container/host IP-forwarding pitfall (not a real-world failure)
- Rephrase exit-node ICMP pitfall: clients can't connect at all

* docs(routing-peers): add example for forward vs input chain policies

A concrete scenario (office subnet behind the peer + Grafana on
TCP/3000 + SSH on TCP/22) showing the two policies needed: a network
resource policy on the forward chain and a peer-to-peer policy on
the input chain.

src/components/NavigationDocs.jsx

@@ -183,6 +183,10 @@ export const docsNavigation = [
         isOpen: false,
         links: [
           { title: 'Concept', href: '/manage/networks' },
+          {
+            title: 'How Routing Peers Work',
+            href: '/manage/networks/how-routing-peers-work',
+          },
           {
             title: 'Use Cases',
             isOpen: false,