help: Write intial help documentation

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

help/C/sending-to-other-apps.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/sending-to-other-apps.page.duck instead and run make in this directory.
+-->
+<page xmlns="http://projectmallard.org/1.0/" type="topic" id="sending-to-other-apps">
+ <info>
+  <link type="guide" xref="index#additional-features"/>
+  <desc>Learn how to send, copy, and drag images from Loupe to other apps.</desc>
+ </info>
+ <title>Sending Images To Other Apps</title>
+ <section>
+  <title>"Open With…" Action</title>
+  <steps>
+   <item>
+    <p>Click the <gui style="menuitem">Open With…</gui> menu item. A dialog with a list of applications will appear.</p>
+   </item>
+   <item>
+    <p>Select the application you want to open the image with.</p>
+   </item>
+  </steps>
+ </section>
+ <section>
+  <title>Copy &amp; Paste</title>
+  <steps>
+   <item>
+    <p>Click the <gui>Copy to Clipboard</gui> (<media type="image" src="figures/edit-copy-symbolic.svg">edit-copy-symbolic</media>) button. A toast notifying you that the image is in your clipboard will appear.</p>
+   </item>
+   <item>
+    <p>Paste your image in your application of choice.</p>
+   </item>
+  </steps>
+ </section>
+ <section>
+  <title>Drag &amp; Drop</title>
+  <steps>
+   <item>
+    <p>Zoom out from the image completely.</p>
+   </item>
+   <item>
+    <p>Click and hold the selected image.</p>
+   </item>
+   <item>
+    <p>Drag the image to your application of choice.</p>
+   </item>
+  </steps>
+ </section>
+</page>