Remove Content Pipeline system from the public repo

Removed by request - it was the one unfinished system in the repo,
and it's the user's call on what represents them publicly. Removed
completely: the README section, docs/content-pipeline.md, and the
two stacks that only existed to back it (postiz, nocodb-content).
Fixed cross-links in dns-naming.md and firewall-and-network.md that
pointed at it (replaced the public-facing-app example with the
still-present Ghost blog), and notifications.md (dropped the
reference, also fixed a leftover 'documented rather than hidden'
phrase in the same paragraph). Stat strip and stack counts updated
(55->53 stacks, 16->15 systems).

docs/notifications.md

@@ -28,7 +28,7 @@ flowchart LR
 The alternative — each app posting directly to a per-app Slack webhook, or a small script per integration — was rejected because it would mean N different message formats, N different places to add a new integration's logic, and no single point to add fleet-wide behavior (like the OS-level checks below) without touching every app individually. One Node-RED instance with a shared router and a shared formatting subflow means a new app's alerts get the same visual treatment as every existing one automatically, and a fleet-wide change (say, adding a severity-based routing rule) happens in one place.
 
 > [!WARNING]
-> **Debt, documented rather than hidden**: of the apps wired into the router, a couple no longer resolve to anything — one was fully decommissioned (see [`docs/registry-and-build.md`](registry-and-build.md) for how stacks get retired) but its detection code was never swept out of the flow, and another's alerts actually travel a completely different path than its own webhook route suggests, making that route effectively dead code. Two newer, genuinely live apps — including the ones described in [`docs/content-pipeline.md`](content-pipeline.md) — aren't wired into either branch yet. Real gaps against the fleet's own documented pattern for onboarding a new app's alerts, left visible here rather than quietly cleaned up before writing this doc, because an accurate picture of the system is more useful than a flattering one.
+> **Real debt in the flow**: of the apps wired into the router, a couple no longer resolve to anything — one was fully decommissioned (see [`docs/registry-and-build.md`](registry-and-build.md) for how stacks get retired) but its detection code was never swept out, and another's alerts actually travel a completely different path than its own webhook route suggests, making that route effectively dead code. A couple of newer, genuinely live apps aren't wired into either branch yet — a real gap against the fleet's own onboarding pattern for a new app's alerts.
 
 ## Message broker