templates: Drop redundant Archive sidebar on the blog listing page

The theme's blog.html shows every post twice on /blog/: once in the
main content list, and again in a sidebar labeled 'Archive'. That's
confusing now that News has been folded in and the list is a bit
longer. The main list already serves as the archive, so just empty
out the sidebar block.

Also fix a stale .gitignore entry left over from when the site lived
under a site/ subdirectory; it was ignoring site/public/ instead of
the public/ dir that zola build actually produces at the repo root.

Assisted-by: AI

Author: cgwalters

.gitignore

@@ -1,4 +1,4 @@
-site/public/
+public/
 .idea
 
 .DS_Store

templates/blog.html

@@ -0,0 +1,9 @@
+{% extends "juice/templates/blog.html" %}
+
+{# The theme's default sidebar just repeats the same post list shown in the
+   main content above as an "Archive" - drop it instead of showing every
+   post twice on the same page. #}
+{% block sidebar %}
+<div class="sidebar">
+</div>
+{% endblock sidebar %}