docs(website): document m:n, --demo, schema sidebar, responsive input

Document the features the main merge shipped: `create m:n relationship`
(relationships ref + build-the-library note), the `--demo` teaching
flag (command-line-options), the Ctrl-O schema sidebar (output-pane,
now .mdx to embed the new cast), and horizontal/two-row input
(assistive-editor).

website/src/content/docs/using-the-playground/command-line-options.md

@@ -27,6 +27,7 @@ rdbms-playground path/to/project
 | `--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/)). |
+| `--demo` | Turn on demonstration mode — a teaching aid that briefly shows an on-screen badge for keys that otherwise leave no visible mark (Tab, Enter, the arrows, and the like). Useful for screencasts and for demonstrating the playground to a class. Off by default. |
 | `--log-file <PATH>` | Write diagnostic logging to `PATH`. |
 | `-h`, `--help` | Print the usage banner and exit. |