Let a gesture name the manual section that shows it

The help sheet says which move does a thing, and the manual has a
picture of the thing being done, but nothing joined the two: a user
reading "Pinch it with two fingers" had no way from there to the GIF of
it.

A GESTURE tag takes an optional `manual:` field naming a heading of
docs/manual/README.md by its anchor. The scan checks every one against
the anchors the bundled page is rendered with and fails when the manual
has no such heading, so renaming a section cannot leave the sheet
linking to the top of the page; gestures-check carries the same failure
into CI. The anchor goes into gesture_book.rs as a new field, and into
docs/gestures.md as a "See it" link to manual/README.md#anchor. The help
sheet draws a "See it" button beside the title of each gesture that has
one, which opens the bundled manual at that section.

The field is additive: a tag without it is unchanged, and no gesture
carries one yet.

ui/dr-ui/src/gestures.rs

@@ -38,6 +38,9 @@ pub struct Row {
     pub touch: String,
     pub pointer: String,
     pub keys: String,
+    /// The manual section that shows the gesture, as a heading anchor; empty
+    /// where there is none, and the sheet then offers no "See it".
+    pub manual: String,
 }
 
 /// Build the sheet's rows from the generated table.
@@ -67,6 +70,7 @@ fn rows_from(gestures: &[Gesture]) -> Vec<Row> {
             touch: g.touch.to_string(),
             pointer: g.pointer.to_string(),
             keys: g.keys.to_string(),
+            manual: g.manual.to_string(),
         });
     }
     out
@@ -83,6 +87,7 @@ mod tests {
             touch: "Tap",
             pointer: "",
             keys: "",
+            manual: "",
         }
     }