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/inserting-and-editing-data.md

@@ -0,0 +1,26 @@
+---
+title: Inserting & editing data
+description: Add, change, and remove rows.
+sidebar:
+  order: 7
+---
+
+:::note[In progress]
+The syntax below is complete and accurate; worked examples on the library
+schema are being added.
+:::
+
+```rdbms-syntax
+insert into <Table> [(<col>[, ...])] [values] (<value>[, ...])
+update <Table> set <col>=<value>[, ...] (where <col>=<value> | --all-rows)
+delete from <Table> (where <col>=<value> | --all-rows)
+```
+
+`update` and `delete` require a `where` clause; to act on every row you must
+opt in explicitly with `--all-rows`. Auto-generated columns (`serial`,
+`shortid`) fill themselves in, so you can leave them out of an `insert`. After
+a write, the affected rows are shown automatically.
+
+`where` accepts more than simple equality — `and` / `or` / `not`,
+comparisons, `like`, `is null`, `in`, and `between`. See
+[Querying & inspecting](/reference/querying-and-inspecting/).