website: remove unused root path on building website (#3248)

We no longer use the `--root` option of `build_website.py` since we have
moved to jqlang.org (from jqlang.github.io/jq where the root was `/jq`).
We don't need the option on local development either since `http.server`
has the `-d` option to specify the directory to serve from.

docs/templates/manual.html.j2

@@ -36,12 +36,12 @@
         <p>
           <em>
             For other versions, see
-            <a href="{{ root }}/manual/v1.7/">1.7</a>,
-            <a href="{{ root }}/manual/v1.6/">1.6</a>,
-            <a href="{{ root }}/manual/v1.5/">1.5</a>,
-            <a href="{{ root }}/manual/v1.4/">1.4</a>,
-            <a href="{{ root }}/manual/v1.3/">1.3</a> or
-            <a href="{{ root }}/manual/dev/">development version</a>.
+            <a href="/manual/v1.7/">1.7</a>,
+            <a href="/manual/v1.6/">1.6</a>,
+            <a href="/manual/v1.5/">1.5</a>,
+            <a href="/manual/v1.4/">1.4</a>,
+            <a href="/manual/v1.3/">1.3</a> or
+            <a href="/manual/dev/">development version</a>.
           </em>
         </p>
         {{ body | markdownify }}
@@ -115,6 +115,6 @@
         {%- endfor %}
       }
     </script>
-    <script src="{{ root }}/js/manual-search.js"></script>
+    <script src="/js/manual-search.js"></script>
   </body>
 </html>