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/library_ui/grid.rs

@@ -195,6 +195,7 @@ pub fn wire<F>(
                     touch: r.touch.into(),
                     pointer: r.pointer.into(),
                     keys: r.keys.into(),
+                    manual: r.manual.into(),
                 })
                 .collect::<Vec<_>>(),
         )));