docs(website): hint feature docs + cast (content for c84a640)

Completes the preceding empty-rename commit: the getting-help "Hints"
section — F1 for a tier-3 teaching hint on the live input, `hint` to
explain the most recent error — with the real rendered block and a cast
showing both (the live-input hint via the demo-mode Ctrl-G→[F1] alias,
since autocast can't send F1, then `hint` on an error). the-assistive-editor
points at F1; CtrlG added to the cast generator's key map.

website/src/content/docs/using-the-playground/getting-help.mdx

@@ -5,6 +5,8 @@ sidebar:
   order: 8
 ---
 
+import Demo from '../../../components/Demo.astro';
+
 The playground has help built in, so you rarely need to leave it.
 
 ```rdbms
@@ -23,5 +25,35 @@ help types
 ```
 Shows the type reference on its own.
 
+## Hints — a deeper nudge
+
+`help` is the reference; **`hint`** is the teaching nudge for what you are doing
+*right now*. A hint goes beyond the always-on colour and the ambient
+[hint line](/using-the-playground/the-assistive-editor/#hints): it explains what
+a command does, shows a worked example, and names the underlying relational
+concept.
+
+<Demo src="/casts/hint.cast" title="F1 hints the command you're typing; after an error, the hint command explains it." />
+
+Press <kbd>F1</kbd> while typing to get a hint for the command you are building:
+
+```
+Hint
+  What:    Add one or more rows to a table.
+  Example: insert into Customers values ('Ann', 'ann@example.io')
+  Concept: A row is one record; each value lines up with a column, in order. Columns typed `serial`/`shortid` fill themselves — leave them out.
+```
+
+After a command fails, the **`hint`** command explains the most recent error —
+what went wrong, an example of the fix, and the concept behind it:
+
+```rdbms
+hint
+```
+
+A submitted command can't see your half-typed line, which is why the live-input
+hint is on <kbd>F1</kbd> while `hint` covers the last error. Both work in simple
+and advanced mode.
+
 Outside the app, run `rdbms-playground --help` for the
 [command-line options](/using-the-playground/command-line-options/).

website/src/content/docs/using-the-playground/the-assistive-editor.mdx

@@ -45,6 +45,10 @@ would fit, the type expected at the current position, and help with the most
 recent error. When the input is empty it reminds you that <kbd>Tab</kbd>
 lists options and `help` lists commands.
 
+For a deeper, teaching-grade hint — what a command does, a worked example, and
+the relational concept behind it — press <kbd>F1</kbd> while typing (or run
+`hint` after an error). See [Getting help](/using-the-playground/getting-help/).
+
 ## Editing the line
 
 Move and edit within the line with the usual keys: <kbd>←</kbd>/<kbd>→</kbd>