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/overview.mdx

@@ -171,7 +171,7 @@ Store credentials for private Docker registries: Docker Hub organizations, GHCR,
 
 ## Automation
 
-### Auto-Update Readiness
+### Auto-Update Policies
 
 Review pending container updates across your fleet with risk badges (`Safe · patch`, `Review · minor`, `Blocked · major`, `Digest rebuild`) and one-line changelog previews on a single board. The hero counts pending updates and tells you how many are ready to apply without human review; stacks with a major version bump are surfaced as a separate count for review. Skipper or Admiral. [Learn more →](/features/auto-update-policies)