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/using-the-playground/the-output-pane.md

@@ -0,0 +1,22 @@
+---
+title: The output pane
+description: Where results appear, and how to scroll back through them.
+sidebar:
+  order: 3
+---
+
+Results appear in the output pane: the structure of a table after you change
+it, the rows from `show data` and after writes (as aligned, box-drawn
+tables), query plans, and command outcomes marked with a ✓ or ✗.
+
+## Scrolling
+
+Use <kbd>PageUp</kbd> and <kbd>PageDown</kbd> to scroll back through earlier
+output. New output snaps the view back to the most recent entry, so you never
+lose your place when you run something.
+
+:::note[Planned]
+A fuller session journal — a scrollable, richly rendered log of the whole
+session that you can save as Markdown — and multiple result tabs are planned
+and not yet available.
+:::