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/export-and-import.md

@@ -0,0 +1,42 @@
+---
+title: Export & import
+description: Package a project to share it, and unpack one you've received.
+sidebar:
+  order: 6
+---
+
+## Export
+
+```rdbms
+export
+```
+
+Packages the project as a single zip. The zip contains the readable files
+(`project.yaml` and `data/`) but **not** the working database (the recipient
+rebuilds it on open) or your private `history.log`. Pass a path to choose
+where it goes:
+
+```rdbms
+export path/to/exercise.zip
+```
+
+## Import
+
+```rdbms
+import path/to/exercise.zip
+```
+
+Unpacks a zip into a new project and switches to it; the database is rebuilt
+from the readable files. Add `as <name>` to choose the target name:
+
+```rdbms
+import path/to/exercise.zip as my-copy
+```
+
+## Sharing recipes
+
+Because a project is plain text plus CSV, sharing it is easy:
+
+- **Git** — each project includes a `.gitignore` that excludes the working
+  database, so a project commits cleanly and diffs sensibly.
+- **Email / file transfer** — send the exported zip; the recipient imports it.