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/src/content/docs/index.mdx

@@ -38,6 +38,10 @@ SQL.
 		Ask the playground to explain any query and it renders the database's
 		plan as an annotated tree — so indexes and scans stop being a mystery.
 	</Card>
+	<Card title="Helpful as you type" icon="pencil">
+		The input field completes commands with <kbd>Tab</kbd>, highlights
+		syntax, flags mistakes before you run them, and hints what comes next.
+	</Card>
 </CardGrid>
 
 ## Start here