docs(peers): add bootstrap via config file guide (#731)

New page covering how to pre-populate default.json so NetBird peers
register with the right settings on first start in IaC, Docker, and
Kubernetes deployments. Documents file location across OSes, common
keys (with guidance to leave PrivateKey empty and treat PreSharedKey
as optional), runtime setup-key injection with worked Docker and
Kubernetes ConfigMap + Secret examples, backup hazards around the
embedded WireGuard private key, and how to verify registration via
netbird status --check startup, --json, and GET /api/peers.

Includes a callout that this workflow is for unattended workloads
only -- end-user devices should still enroll via the SSO flow so
Zero Trust can re-verify the user identity.

Linked from Manage NetBird -> Peers, right after Setup Keys.

src/components/NavigationDocs.jsx

@@ -96,6 +96,10 @@ export const docsNavigation = [
             title: 'Setup Keys',
             href: '/manage/peers/register-machines-using-setup-keys',
           },
+          {
+            title: 'Bootstrap via Config File',
+            href: '/manage/peers/bootstrap-via-config-file',
+          },
           { title: 'Browser Client', href: '/manage/peers/browser-client' },
           { title: 'SSH', href: '/manage/peers/ssh' },
           { title: 'Lazy Connections', href: '/manage/peers/lazy-connection' },