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/constraints.md

@@ -0,0 +1,30 @@
+---
+title: Constraints
+description: NOT NULL, UNIQUE, DEFAULT, and CHECK constraints on columns.
+sidebar:
+  order: 6
+---
+
+:::note[In progress]
+The syntax below is complete and accurate; worked examples on the library
+schema are being added.
+:::
+
+Constraints keep your data valid. Declare them in the column spec at
+`create table` / `add column` time, or change them on an existing column:
+
+```rdbms-syntax
+add constraint not null to <Table>.<col>
+add constraint unique to <Table>.<col>
+add constraint default <value> to <Table>.<col>
+add constraint check (<expr>) to <Table>.<col>
+drop constraint (not null | unique | default | check) from <Table>.<col>
+```
+
+Adding a constraint to a column that already has rows is checked first — if
+existing data would violate it, the change is refused and the offending rows
+are shown.
+
+In advanced mode these are the standard column- and table-level constraints
+in `CREATE TABLE` / `ALTER TABLE`. The primary key is fixed at creation (see
+[Tables](/reference/tables/)).