docs: Add widget overview

Part-of: <https://gitlab.gnome.org/Incubator/loupe/-/merge_requests/142>

src/main.rs

@@ -17,6 +17,35 @@
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
 
+/*!
+# Loupe Image Viewer
+
+Code documentation
+
+# Widget Structure
+
+Simplified widget arrangement including the most important widgets.
+
+-   [`LpWindow`]
+    -   [`AdwFlap`]
+        -   [`LpPropertiesView`]
+        -   [`LpImageView`]
+            -   [`LpSlidingView`]
+                -   [`LpImagePage`]
+                    -   [`GtkScrolledWindow`]
+                        -   [`LpImage`]
+                -   …
+
+[`AdwFlap`]: adw::Flap
+[`GtkScrolledWindow`]: gtk::ScrolledWindow
+[`LpImagePage`]: widgets::LpImagePage
+[`LpImageView`]: widgets::LpImageView
+[`LpImage`]: widgets::LpImage
+[`LpPropertiesView`]: widgets::LpPropertiesView
+[`LpSlidingView`]: widgets::LpSlidingView
+[`LpWindow`]: window::LpWindow
+*/
+
 use gettextrs::*;
 use gtk::gio::{self, prelude::*};