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>

docs/index.md

@@ -66,7 +66,7 @@ out of it; examples use placeholders like `<workspace>`, `<project>`, and
     Project-based Python isolation — local `.venv`, Makefile, or Docker — and
     local version management with pyenv.
 
--   :material-rocket-launch: __[Deploy](deploy/)__
+-   :material-rocket-launch: __[Deploy](deploy.md)__
 
     ---