Fall back to archive.org for HTML Help Workshop †

Because [Cool URIs don't change](https://www.w3.org/Provider/Style/URI.html)

- https://learn.microsoft.com/en-us/answers/questions/265752/htmlhelp-workshop-download-for-chm-compiler-instal

Fixes #536.

Signed-off-by: Roger Sheen <[email protected]>

resources/external-links.ditamap

@@ -343,12 +343,12 @@
     </keydef>
     <keydef
       keys="download.html-help-workshop"
-      href="http://msdn.microsoft.com/en-us/library/windows/desktop/ms669985%28v=vs.85%29.aspx"
+      href="http://web.archive.org/web/20160201063255/http://download.microsoft.com/download/0/A/9/0A939EF6-E31C-430F-A3DF-DFAE7960D564/htmlhelp.exe"
       scope="external"
       format="html"
     >
       <topicmeta>
-        <linktext>msdn.microsoft.com</linktext>
+        <linktext>web.archive.org</linktext>
       </topicmeta>
     </keydef>
     <keydef

topics/prerequisite-software.dita

@@ -59,8 +59,9 @@
       <p>Depending on the type of output that you want to generate, you might need the following applications:
         <dl>
           <dlentry>
-            <dt>Microsoft Help Workshop</dt>
-            <dd>Required for generating HTML help. You can download the Help Workshop from
+            <dt>HTML Help Workshop</dt>
+            <dd>Microsoft no longer provides the software required for generating Compiled HTML Help (.chm) files. You
+              can download an archived copy of the HTML Help Workshop from the Internet Archive’s Wayback Machine at
               <xref keyref="download.html-help-workshop"/>.</dd>
           </dlentry>
           <dlentry>