wiki: Document the DnD edge view scroll gesture and config

wiki/Configuration:-Gestures.md

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+### Overview
+
+<sup>Since: next release</sup>
+
+The `gestures` config section contains gesture settings.
+For an overview of all niri gestures, see the [Gestures](./Gestures.md) wiki page.
+
+Here's a quick glance at the available settings along with their default values.
+
+```kdl
+gestures {
+    dnd-edge-view-scroll {
+        trigger-width 30
+        delay-ms 50
+        max-speed 1500
+    }
+}
+```
+
+### `dnd-edge-view-scroll`
+
+Scroll the tiling view when moving the mouse cursor against a monitor edge during drag-and-drop (DnD).
+Also works on a touchscreen.
+
+This will work for regular drag-and-drop (e.g. dragging a file from a file manager), and for window interactive move when targeting the tiling layout.
+
+The options are:
+
+- `trigger-width`: size of the area near the monitor edge that will trigger the scrolling, in logical pixels.
+- `delay-ms`: delay in milliseconds before the scrolling starts.
+Avoids unwanted scrolling when dragging things across monitors.
+- `max-speed`: maximum scrolling speed in logical pixels per second.
+The scrolling speed increases linearly as you move your mouse cursor from `trigger-width` to the very edge of the monitor.
+
+```kdl
+gestures {
+    // Increase the trigger area and maximum speed.
+    dnd-edge-view-scroll {
+        trigger-width 100
+        max-speed 3000
+    }
+}
+```