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/reference/tables.md

@@ -6,8 +6,8 @@ sidebar:
 ---
 
 A table is the core building block. This page covers creating and removing
-tables; changing a table's columns after it exists is covered under Columns
-in the Reference.
+tables; changing a table's columns after it exists is covered in
+[Columns](/reference/columns/).
 
 ## Create a table (simple mode)
 
@@ -62,7 +62,8 @@ create table authors (
 ```
 
 The advanced form also accepts table-level constraints and inline foreign
-keys — see the Constraints and Relationships references.
+keys — see [Constraints](/reference/constraints/) and
+[Relationships](/reference/relationships/).
 
 **Syntax**