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/astro.config.mjs

@@ -33,6 +33,7 @@ export default defineConfig({
 			// frontmatter.
 			sidebar: [
 				{ label: 'Getting started', items: [{ autogenerate: { directory: 'getting-started' } }] },
+				{ label: 'Using the playground', items: [{ autogenerate: { directory: 'using-the-playground' } }] },
 				{ label: 'Guides', items: [{ autogenerate: { directory: 'guides' } }] },
 				{ label: 'Reference', items: [{ autogenerate: { directory: 'reference' } }] },
 				{ label: 'Concepts', items: [{ autogenerate: { directory: 'concepts' } }] },