[about.html] Add para with back-link to source code (#18)

Add a paragraph with a back-link to the source code repository, as [planned two days ago](#15 (comment)).

chumweb/www/views/pages/about.html

@@ -8,12 +8,18 @@
     <header>
         <h1>Chum Web</h1>
     </header>
+    <h2>Chum Web Generator</h2>
+    <p>
+        This website is built by the Chum Web Generator <a href="{{config.source_code_url}}">(view source repository)</a>.
+        To gather necessary information, it will make requests with the User-Agent <code>{{config.user_agent}}</code>.
+    </p>
+    <h2>SailfishOS:Chum</h2>
     <p>
         The SailfishOS:Chum community repository provides a collection of applications, tools and libraries for the
         mobile operating system <a href="https://sailfishos.org/">Sailfish OS</a>, compiled for
         various hardware architectures and OS release versions.
     </p>
-    <h2>Goal</h2>
+    <h3>Goal</h3>
     <p>
         The ambition is to become the principal software distribution platform for Sailfish OS.
     </p>
@@ -37,9 +43,10 @@ <h2>Goal</h2>
         dependencies ("dependency chains") which is crucial for keeping the software supply chains of complex
         packages up-to-date.
     </p>
-    <h2>Location</h2>
+    <h3>Location</h3>
     <p>
-        The SailfishOS:Chum repository is located at the
-        <a href="https://build.sailfishos.org/project/show/sailfishos:chum">the Sailfish OS OBS</a>.
+        The SailfishOS:Chum repository proper is located at the
+        <a href="https://build.sailfishos.org/project/show/sailfishos:chum">the Sailfish OS OBS</a> and
+        <a href="https://github.com/sailfishos-chum/main">its project site is at GitHub</a>.
     </p>
 {% endblock main %}