docs: sort the top-level directories into five sections by what the reader wants

Every docs directory moves under 1-getting-started, 2-handbook,
4-implementation or 5-project; the options reference (section 3) comes from
the website and has no directory here. trust-boundary stays whole under the
handbook for now. web-ui, swarm and tools split between handbook and
implementation, and the command-line references move to
2-handbook/reference. Links, path references in code and nix, and the docs
index follow the moves; no page content changes.

frontend/packages/shared/src/tabs/tabs.css

@@ -6,7 +6,7 @@
    so this inherits theme-swap safety for free. Call sites keep a thin
    variant class for size/spacing deltas; richer affordances (count
    pills, the dashboard's responsive overflow ⋮ menu) layer on top in the
-   per-page CSS. See docs/web-ui/css-vars.md. */
+   per-page CSS. See docs/4-implementation/web-ui/css-vars.md. */
 
 .hive-tabbar {
   display: flex;

frontend/packages/shared/src/tabs/tabs.js

@@ -15,7 +15,7 @@
 //
 // On show: toggles `.hive-tab--active` + `aria-selected` on each control
 // and `hidden` on each panel, then fires `onShow(id)` for per-tab
-// side-effects (lazy loads, etc.). See docs/web-ui/css-vars.md.
+// side-effects (lazy loads, etc.). See docs/4-implementation/web-ui/css-vars.md.
 
 /**
  * @param {Element|null} tabbar  container holding the `[data-tab]` controls