help: Write intial help documentation

Part-of: <https://gitlab.gnome.org/GNOME/Incubator/loupe/-/merge_requests/248>

help/C/opening-images.page

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+<?xml version="1.0" encoding="utf-8"?>
+<!--
+  DO NOT EDIT THIS FILE!
+
+  This file is autogenerated with ducktype.
+  Edit duck/opening-images.page.duck instead and run make in this directory.
+-->
+<page xmlns="http://projectmallard.org/1.0/" type="topic" id="opening-images">
+ <info>
+  <link type="guide" xref="index#getting-started"/>
+  <desc>Open and browse through pictures via the file chooser, file manager, or drag and drop.</desc>
+ </info>
+ <title>Opening Images</title>
+ <section>
+  <title>Opening Files</title>
+  <section>
+   <title>From The File Chooser</title>
+   <p>Loupe allows users to pick images with the file chooser at any time.</p>
+   <steps>
+    <item>
+     <p>Click the <gui>Open Files…</gui> button, the <gui style="menuitem">Open…</gui> menu item, or press <keyseq><key>Ctrl</key><key>O</key></keyseq>. The <gui>Open Image</gui> file dialog will appear.</p>
+    </item>
+    <item>
+     <p>Select the image you want to open, then click <gui>Open</gui>.</p>
+    </item>
+   </steps>
+   <p>Loupe will open with the selected image showing, and the other images in the directory can be reached by browsing.</p>
+  </section>
+  <section>
+   <title>From The File Manager</title>
+   <p>Users can also open images in Loupe directly from their file manager.</p>
+   <steps>
+    <item>
+     <p>Open your preferred file manager application.</p>
+    </item>
+    <item>
+     <p>Navigate to the image you want to view in Loupe.</p>
+    </item>
+    <item>
+     <p>If Loupe is set as your default image viewer, use the file manager's function for opening files.</p>
+    </item>
+    <item>
+     <p>If Loupe is not your default image viewer, you will need to select <gui>Open With…</gui> or similar in the file manager, and pick Loupe.</p>
+    </item>
+   </steps>
+  </section>
+  <section>
+   <title>Drag &amp; Drop</title>
+   <p>Loupe also accepts local image files dragged from other applications.</p>
+   <steps>
+    <item>
+     <p>In the source application, begin dragging the image.</p>
+    </item>
+    <item>
+     <p>Drag the image over the Loupe main window. A blue highlighted area should appear.</p>
+    </item>
+    <item>
+     <p>Drop the image while it's still hovered over the main window.</p>
+    </item>
+   </steps>
+  </section>
+ </section>
+ <section>
+  <title>Browsing</title>
+  <p>When opening an image from the file chooser or file manager, Loupe will load other files
+  in the parent directory of the image and sort them in alphabetical order. Users can
+  browse the other images in the directory with the <gui>Previous Image</gui> (<media type="image" src="figures/go-previous-symbolic.svg">go-previous-symbolic</media>)
+  and <gui>Next Image</gui> (<media type="image" src="figures/go-next-symbolic.svg">go-next-symbolic</media>) buttons.
+  Keyboard users can also use the following shortcuts:</p>
+  <list>
+   <item>
+    <p><key>→</key> to go to the next image.</p>
+   </item>
+   <item>
+    <p><key>←</key> to go to the previous image.</p>
+   </item>
+   <item>
+    <p><key>Home</key> to jump to the first image in the directory.</p>
+   </item>
+   <item>
+    <p><key>End</key> to jump to the last image in the directory.</p>
+   </item>
+  </list>
+ </section>
+</page>