Revert "general: Remove help-related code"

This reverts commit 2965799374.

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

help/C/index.page

@@ -0,0 +1,19 @@
+<page xmlns="http://projectmallard.org/1.0/"
+      type="guide" style="task"
+      xmlns:its="http://www.w3.org/2005/11/its"
+      id="index">
+
+  <info>
+    <desc>Help for Loupe.</desc>
+    <title type='link' role="trail"><media its:translate="no" type="image" src="figures/org.gnome.eog-symbolic.svg"/></title>
+    <title type='link'>Loupe</title>
+    <title type='text'>Loupe</title>
+    <credit type="author copyright">
+      <name>Christopher Davis</name>
+      <email>christopherdavis@gnome.org</email>
+      <years>2020</years>
+    </credit>
+  </info>
+
+<title>Loupe</title>
+</page>

help/meson.build

@@ -0,0 +1,8 @@
+sources = [
+  'index.page'
+]
+
+gnome.yelp(
+  meson.project_name(),
+  sources: sources,
+)