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/relationships.md

@@ -0,0 +1,38 @@
+---
+title: Relationships
+description: Declare and remove foreign-key relationships between tables.
+sidebar:
+  order: 4
+---
+
+:::note[In progress]
+The syntax below is complete and accurate; worked examples on the library
+schema are being added.
+:::
+
+A relationship is a foreign key from a child table to a parent's primary
+key. In simple mode:
+
+```rdbms-syntax
+add 1:n relationship [as <Name>]
+  from <Parent>.<col> to <Child>.<col>
+  [on delete <action>] [on update <action>]
+  [--create-fk]
+drop relationship <Name>
+```
+
+`<action>` is `cascade`, `set null`, or `restrict`. `--create-fk` adds the
+child column if it doesn't exist yet.
+
+**Compound keys.** To reference a parent's compound primary key, list the
+columns on each side, matched in order:
+
+```rdbms-syntax
+add 1:n relationship from <Parent>.(<a>, <b>) to <Child>.(<x>, <y>)
+```
+
+In advanced mode, relationships are declared with `FOREIGN KEY … REFERENCES`
+in `CREATE TABLE` or `ALTER TABLE`.
+
+See also [Indexes](/reference/indexes/) and [Types](/reference/types/) (why a
+foreign-key column's type can differ from the key it references).