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/javascripts/mermaid-init.js

@@ -0,0 +1,29 @@
+// render mermaid diagrams (emitted as <div class="mermaid">SOURCE</div>).
+// render(id, src) is used directly and each block is processed once.
+(function () {
+  var inited = false;
+  var seq = 0;
+  function boot() {
+    if (typeof mermaid === "undefined") return;
+    if (!inited) {
+      mermaid.initialize({ startOnLoad: false, theme: "dark", securityLevel: "loose" });
+      inited = true;
+    }
+    document.querySelectorAll("div.mermaid").forEach(function (el) {
+      if (el.dataset.mmdDone) return;
+      var src = el.textContent.trim();
+      if (!src) return;
+      el.dataset.mmdDone = "1";
+      mermaid.render("mmd-" + seq++, src).then(function (out) {
+        el.innerHTML = out.svg;
+      }).catch(function () {
+        delete el.dataset.mmdDone;
+      });
+    });
+  }
+  if (window.document$ && typeof window.document$.subscribe === "function") {
+    window.document$.subscribe(boot);
+  } else {
+    document.addEventListener("DOMContentLoaded", boot);
+  }
+})();