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/guides/build-the-library.md

@@ -102,6 +102,15 @@ add 1:n relationship as loans_book from books.book_id to loans.book_id on delete
 add 1:n relationship as loans_member from members.member_id to loans.member_id on delete cascade
 ```
 
+:::note
+We build the bridge by hand here because `loans` carries its own columns
+(`loaned_on`, `returned_on`) — it records *when* each borrowing happened, not
+just *that* it did. For a pure link with no extra columns, the
+`create m:n relationship` command builds the junction table and both
+relationships in one step — see
+[Many-to-many relationships](/reference/relationships/#many-to-many-relationships).
+:::
+
 You can confirm all three relationships at once:
 
 ```rdbms