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`hint`-command note (superseded by the hint panel).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/using-the-playground/getting-help.md

@@ -25,9 +25,3 @@ Shows the type reference on its own.
 
 Outside the app, run `rdbms-playground --help` for the
 [command-line options](/using-the-playground/command-line-options/).
-
-:::note[Planned]
-A `hint` command for contextual help on the current input or the last error
-is planned and not yet available — but the input already shows hints as you
-type (see [The assistive editor](/using-the-playground/the-assistive-editor/)).
-:::

website/src/content/docs/using-the-playground/the-output-pane.mdx

@@ -36,6 +36,5 @@ show more detail and is marked with a coloured border;
 
 :::note[Planned]
 A fuller session journal — a scrollable, richly rendered log of the whole
-session that you can save as Markdown — and multiple result tabs are planned
-and not yet available.
+session that you can save as Markdown — is planned and not yet available.
 :::