Rewrite and publish Networking concept page for rc44

[kcmartin]

Rewrites the draft Networking page into a concept hub, matching Lifecycle and Persistence, Checkpoints, and Services: explain the model, link out for procedure, don't duplicate what other pages own.

What changed

• Reframed the ~260-line how-to draft into a ~130-line concept hub: two ways in (Sprite URL, sprite proxy) and one policy governing what goes out.
• Corrected a real bug: the draft claimed "full network access, nothing restricted." Outbound is actually governed by a DNS-based egress policy. The page now documents the model: allow-list rules, the defaults include, wildcard specificity, REFUSED on denied domains, raw-IP and private-IP blocking, live reload, read-only from inside, changed only via the API.
• Fixed stale/invalid commands: dropped sprite exec -detachable, sprite ps, and sprite url --public; used current sprite url update --auth and the verified .sprites.dev domain (the draft and working-with-sprites disagreed on .sprites.app).
• Removed the SDK tabs and the cookbook (dev server, Postgres, multi-service) that duplicated Services and Working with Sprites.
• Added the URL-vs-proxy distinction table and the sprite exec port auto-forwarding behavior.
• Linked Connectors as the distinct "reach external APIs with brokered credentials" mechanism, not folded in.
• Published: draft removed, publishedDate + author set, added to the Concepts sidebar between Lifecycle and Services.

Command detail stays on Working with Sprites and the CLI Commands reference; this page links to both. base-images already links here for egress-policy detail, which this page now owns.

[github-actions]

Preview Deployment

Name	URL
Preview	https://pr-204-superfly-sprites-docs.fly.dev

Commit: f3fc68f

[github-actions]

Lighthouse Results

URL	Performance	Accessibility	Best Practices	SEO
/	🟠 83	🟢 100	🟢 96	🟢 100
/concepts/networking/	🟢 97	🟢 100	🟢 100	🟢 100

[github-actions]

E2E Test Results

✅ Tests success

Ran against: https://pr-204-superfly-sprites-docs.fly.dev

[jbellerb]

Only real issue is the deprecation.