deploy: mermaid flow diagram, single page with tabs, section rework

Collapse the deploy sub-pages back into one deploy.md (tabs, not separate
pages), add a working mermaid flow diagram, and rework the top per feedback.

- Mermaid: enable via superfences fence_div_format (the fence_code_format
  <pre><code> wrapper broke mermaid's render — div format fixes it); vendor
  mermaid.min.js locally (no CDN dependency for a self-hosted site) + a small
  init that renders each block once and survives instant-nav.
- Content tabs Compose / Dockerfile / Secrets hold the three pieces together
  on one page (inline comments in the code, no annotation +).
- 'What can run here' tip moved above the flow; section renamed
  'Preparing for Deploy' with the 'you don't run deploy commands' point folded
  sparsely into the intro (standalone note removed); 'svc never changes'
  called out.
- Flow diagram (below the tabs): build group now includes our libraries; git
  -> staging shown as develop-if-gitflow (dotted); deployment (no MR-to-main)
  -> docker image built + run on the fleet.
- Revert inbound links to deploy.md.

Verified in-browser: diagram renders with all nodes, tabs switch, section
order correct; mkdocs build --strict clean.

Signed-off-by: disqualifier <dev@disqualifier.me>

mkdocs.yml

@@ -6,6 +6,10 @@ copyright: rethink development (handbook)
 extra_css:
   - stylesheets/extra.css
 
+extra_javascript:
+  - javascripts/mermaid.min.js
+  - javascripts/mermaid-init.js
+
 theme:
   name: material
   language: en
@@ -35,7 +39,11 @@ markdown_extensions:
   - toc:
       permalink: true
   - pymdownx.details
-  - pymdownx.superfences
+  - pymdownx.superfences:
+      custom_fences:
+        - name: mermaid
+          class: mermaid
+          format: !!python/name:pymdownx.superfences.fence_div_format
   - pymdownx.tabbed:
       alternate_style: true
   - pymdownx.highlight:
@@ -55,10 +63,4 @@ nav:
   - Standards: standards.md
   - Workflow: workflow.md
   - Virtual environments: environments.md
-  - Deploy: deploy/index.md
-
-# Deploy sub-pages are reached from the Deploy hub's cards, not the global nav.
-not_in_nav: |
-  /deploy/compose.md
-  /deploy/dockerfile.md
-  /deploy/secrets.md
+  - Deploy: deploy.md