docs: rewrite Networks page as a teaching guide (#779)

* docs: rewrite Networks page as a teaching guide

Rework /manage/networks from a reference-style concept page into a
structured teaching guide: mental model, the four building blocks
(Network, Resource, Access policy, Routing peer), how a packet reaches
a resource, and an end-to-end walkthrough for reaching two internal
apps with Zero Trust access by default.

- Add a production checklist (HA, monitoring, masquerade, internal DNS,
  routing-peer access) and a clear "Networks or Network Routes?" split:
  Networks now covers every remote-access scenario except exit nodes.
- Add worked-example and resource-list screenshots.
- how-routing-peers-work: point site-to-site at Networks, describe
  Routing Peer DNS Resolution as on by default, and restore the
  0.59.x domain-resolution compatibility note.
- Rename the sidebar entry from "Concept" to "Overview".

* image organization into /networks dir

* fix embedded images in networks/index.mdx

* docs: note no inbound ports and Linux-only masquerade on Networks page

---------

Co-authored-by: TechHutTV <brandon@techhut.tv>

src/components/NavigationDocs.jsx

@@ -190,7 +190,7 @@ export const docsNavigation = [
         title: 'Networks',
         isOpen: false,
         links: [
-          { title: 'Concept', href: '/manage/networks' },
+          { title: 'Overview', href: '/manage/networks' },
           {
             title: 'How Routing Peers Work',
             href: '/manage/networks/how-routing-peers-work',