Fix #59: Footer /about link points to missing route

Change href="/about" to href="/about.html" in footer.
About page exists as about.html but the link pointed to /about
which 404s on plain static servers and nginx without try_files.

Works under python3 -m http.server and deployed nginx.

Closes #59

index.html

@@ -680,7 +680,7 @@ html, body {
 
   <!-- Footer -->
   <footer id="footer">
-    <a href="/about" aria-label="About The Door">about</a>
+    <a href="/about.html" aria-label="About The Door">about</a>
     <button id="safety-plan-btn" aria-label="Open My Safety Plan">my safety plan</button>
     <button id="clear-chat-btn" aria-label="Clear chat history">clear chat</button>
   </footer>