docs: reorganize navigation for feature discoverability (#1147)

Restructures the Documentation tab so each group answers one operator
question.

- Split the 12-page "Stacks & Deployments" into Stacks (per-stack work)
  and Deployment (the act of deploying); promote Resources Hub to a
  standalone item.
- Dissolve the 2-page "Platform" junk drawer: Sidebar moves to Stacks,
  Host Console moves to Fleet.
- Rename "Fleet & Multi-Node" to "Fleet"; absorb Node Compatibility
  from Reference. Move Scheduled Operations from Fleet to Automation
  (now a 4-page group covering Scheduled Ops, Auto-Update, Auto-Heal,
  Webhooks).
- Clean up the Reference tab: drop misplaced node-compatibility, move
  root-level security.mdx into reference/, delete the orphan
  reference/verifying-images.mdx after porting its Available Tags
  table into operations/verifying-images.mdx.
- Reorder top-level groups: Operations moves above Reference.
- Rename two misleading page titles: "Deploy Progress Modal" becomes
  "Deploy Progress" (drops the UI implementation leak); "Auto-Update
  Readiness" becomes "Auto-Update Policies" (matches filename and
  sibling "Auto-Heal Policies").

Verified: docs.json parses as valid JSON, 59 disk .mdx files match
59 nav entries with zero orphans and zero broken refs.

docs/features/node-compatibility.mdx

@@ -60,7 +60,7 @@ Every Sencho release ships with a static list of capabilities. The current list
 | `global-logs` | Global log viewer (always available) |
 | `system-stats` | Host system statistics (always available) |
 | `fleet` | Fleet management view |
-| `auto-updates` | Auto-Update Readiness policies |
+| `auto-updates` | Auto-Update policies |
 | `labels` | Stack labels |
 | `webhooks` | Webhook triggers |
 | `network-topology` | Network topology view |