docs(website): expand the SQL-echo section; prune over-promised notes

Rewrite "Seeing the SQL behind a command" with the learning framing,
a grounded ALTER TABLE example, and the sql-echo cast. Drop the
"multiple result tabs" promise (won't-do on main) and the planned
`hint`-command note (superseded by the hint panel).

website/src/content/docs/getting-started/modes.mdx

@@ -58,7 +58,30 @@ That runs just this one line as SQL; you stay in simple mode afterwards.
 
 ## Seeing the SQL behind a command
 
-When you run a simple-mode command in advanced mode, the playground prints
-the equivalent SQL beneath your command. It is a built-in way to learn how
-the friendly commands map onto real SQL — the same statements you could type
-yourself in advanced mode.
+Run a **simple-mode command while in advanced mode** and the playground prints
+the equivalent SQL beneath it, tagged `Executing SQL:`. This is one of the most
+useful ways the playground teaches: you write the friendly, readable command,
+and immediately see the real SQL statement it stands for — the same statement
+you could have typed yourself.
+
+It turns every command into a small SQL lesson. Add a column the easy way and
+watch the `ALTER TABLE` it maps to:
+
+```rdbms
+add column to books: title (text)
+```
+
+```
+Executing SQL: ALTER TABLE books ADD COLUMN title text
+```
+
+The payoff grows with the command. A single `create m:n relationship` — the
+one-line way to
+[link two tables many-to-many](/reference/relationships/#many-to-many-relationships)
+— expands to an entire junction table: two foreign-key columns, a compound
+primary key, and two cascading foreign keys, all spelled out in the echo.
+
+<Demo src="/casts/sql-echo.cast" title="Simple-mode commands run in advanced mode each echo the SQL they run — ending with the m:n command expanding to a full junction table." />
+
+Because the echo is exactly what runs, it doubles as a recipe: read it, copy
+it, tweak it, and run your own version in advanced mode.