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/command-line-options.md

@@ -0,0 +1,44 @@
+---
+title: Command-line options
+description: How to start the playground, open a project, and the flags you can pass.
+sidebar:
+  order: 1
+---
+
+Start the playground with no arguments and it opens a fresh, automatically
+named temporary project so you can begin straight away:
+
+```sh
+rdbms-playground
+```
+
+To open an existing project, give its path:
+
+```sh
+rdbms-playground path/to/project
+```
+
+## Options
+
+| Option | Effect |
+|---|---|
+| `--resume` | Reopen the most recently used project. Errors if there is none; cannot be combined with a project path. |
+| `--data-dir <PATH>` | Use `PATH` as the data root instead of the OS-standard location for this run. |
+| `--mode <simple\|advanced>` | Start in this input mode, overriding the project's stored mode (precedence: `--mode` > stored > simple). |
+| `--theme <light\|dark>` | Force a theme instead of auto-detecting from the terminal. |
+| `--no-undo` | Disable the undo machinery for this run — no snapshot is taken before each change (see [Undo, redo & history](/using-the-playground/undo-and-history/)). |
+| `--log-file <PATH>` | Write diagnostic logging to `PATH`. |
+| `-h`, `--help` | Print the usage banner and exit. |
+
+## Examples
+
+```sh
+# Reopen wherever you left off, in advanced mode
+rdbms-playground --resume --mode advanced
+
+# Keep a course's projects in one folder
+rdbms-playground --data-dir ~/db-course
+```
+
+Where projects are stored, and what a project contains, is covered in
+[Projects and storage](/concepts/projects-and-storage/).