Link fifty gestures on the help sheet to the manual section that shows them

The sheet could now offer "See it", but no gesture said where to look.

Every GESTURE tag whose move the manual describes names that section:
the white-balance picker, zoom and pan, masks, undo and snapshots,
export, colour labels and ratings, selection, collections, the People
page, thumbnail size. Fifty of the fifty-one; the one left, putting a
single control back to its default, has no section and is too small to
earn one.

Three important gestures had nowhere to land, so the manual gains three
short sections, without pictures for now: Bursts (opening a folded
burst, choosing the frame it shows, and the eyes-open filter), Moving
between photographs (the roll, the arrows and A/D in develop) and
Copying settings (Copy, Paste, Paste to N, and choosing what a copy
carries). The page, the gesture book and docs/gestures.md are
regenerated from them.

ui/dr-ui/ui/identity.slint

@@ -193,6 +193,7 @@ component FaceCell inherits Rectangle {
             //            children, and on the same person a decade apart, so
             //            splitting is as prominent as merging. A tool that can
             //            only merge makes its own errors permanent.
+            //   manual:  people
             TouchArea {
                 clicked => { root.toggle-pick(); }
             }
@@ -218,6 +219,7 @@ component FaceCell inherits Rectangle {
             // suggestion they sit beside, and `IconButton`'s fallback would
             // announce them as "check" and "cross" — two icon names that say
             // nothing about which person is being ruled on.
+            //   manual:  people
             if !face.confirmed: IconButton {
                 icon: "check";
                 label: "Confirm this face";
@@ -639,6 +641,7 @@ export component IdentityScreen inherits Rectangle {
                 //   why:     This is the point of having identified anybody.
                 //            Without it the screen is a filing cabinet with no
                 //            drawer handles.
+                //   manual:  people
                 if root.selected-person >= 0: Button {
                     text: "Show photos";
                     clicked => { root.show-photos(root.selected-person, false); }
@@ -678,6 +681,7 @@ export component IdentityScreen inherits Rectangle {
                 //            answers for this library without writing
                 //            anything; names, confirmations and the groups you
                 //            have set aside are kept whatever the dials say.
+                //   manual:  people
                 Button {
                     text: "Grouping…";
                     active: root.grouping-open;