feat: add "Using the playground" section + Reference skeleton

Restructure the docs into five top-level sections, splitting the
application you drive from the database language you build with.

- New "Using the playground" section: command-line options; the assistive
  editor (completion, highlighting, [ERR]/[WRN] indicator, hints, in-line
  editing); the output pane (scrolling); projects (save/load/new/rebuild);
  undo/redo & history; export & import; clipboard; getting help. Grounded in
  the in-app help/usage and ADR-0003/0022/0027.
- Reference: seed the remaining topic pages (Columns, Relationships,
  Indexes, Constraints, Inserting & editing data, Querying & inspecting)
  with real syntax synopses; worked examples to follow.
- Surface the assistive editor on the landing page and in Getting started;
  restore cross-links now that targets exist.

Plan + STYLE updated to the five-section structure. 24 pages, build green,
links resolve, content clean; planned features carry "planned" callouts.

website/STYLE.md

@@ -41,11 +41,17 @@ Status tags used below: **[DECIDED]** (binding or settled) ·
 
 ## Structure  [DECIDED]
 
-Pragmatic, Diátaxis-influenced split (four top-level sidebar sections,
+Pragmatic, Diátaxis-influenced split (five top-level sidebar sections,
 autogenerated per directory under `src/content/docs/`):
 
 - **Getting started** — install, first project, simple vs. advanced, the
   example database.
+- **Using the playground** — *the application you drive*, kept distinct from
+  the database-language Reference: command-line options, the assistive editor
+  (completion / highlighting / `[ERR]`/`[WRN]` indicator / hints / in-line
+  editing), the output pane (scrolling), projects (save/load/new/rebuild),
+  undo & history, export & import, clipboard, getting help. (ADR-0003's
+  "app-level commands" + ADR-0022/0027 typing assistance + the CLI.)
 - **Guides** — task-oriented how-tos. *These are the most important didactic
   content and will be iterated for teaching quality before publication.*
 - **Reference** — the exhaustive command/SQL/type surface. **Page