docs: add a Performance troubleshooting page (#930)

Add a decision-flow guide for "NetBird feels slow" that helps a reader
find whether the tunnel, their own connection, a routing peer, or the app
is the real cause, instead of assuming NetBird is at fault.

The page leads with the path traffic takes, a one-minute Quick test that
resolves the two most common causes (a relayed peer, or the local
network), then a Start here checklist that links down to detail sections:
checking the connection with netbird status -d, setting a baseline with
iperf3 in both directions, isolating the slow hop, ruling out packet size
and inspecting firewalls, and separating startup delays from throughput.

Add a reusable PathFlow component that draws the hop-by-hop path as a
labelled icon flow, used for the overview, the routing-peer example, and
the recap. Wire the page into the docs sidebar and the troubleshooting
hub.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/components/NavigationDocs.jsx

@@ -1062,6 +1062,10 @@ export const docsNavigation = [
                                 title: 'Resource Connectivity',
                                 href: '/help/troubleshooting-resource-connectivity',
                             },
+                            {
+                                title: 'Performance',
+                                href: '/help/troubleshooting-performance',
+                            },
                             {
                                 title: 'Reverse Proxy',
                                 href: '/manage/reverse-proxy/troubleshooting',