Bring back /private and turn it into a redirect

- Streamline/cleanup some article redirecting code

_categories/private.md

@@ -0,0 +1,7 @@
+---
+title: Private Articles
+permalink: /private/
+layout: article
+icon: lock
+redirect_to: https://lg-public.pages.production.gitlab.login.gov/identity-internal-handbook/
+---

_includes/article_url.txt

@@ -1,9 +1,13 @@
+{% comment %}
+Includes:
+- article: page to link to (or directly to its .redirect_to)
+{% endcomment %}
 {%- if include.article.redirect_to -%}
-  {%- if include.redirect_to contains '://' -%}
-    {{ article.redirect_to }}
+  {%- if include.article.redirect_to contains '://' -%}
+    {{ include.article.redirect_to }}
   {%- else -%}
-    {{ article.redirect_to | prepend: site.baseurl }}
+    {{ include.article.redirect_to | prepend: site.baseurl }}
   {%- endif -%}
 {%- else -%}
-  {{ article.url | prepend: site.baseurl }}
+  {{ include.article.url | prepend: site.baseurl }}
 {%- endif -%}

index.md

@@ -27,8 +27,8 @@ to [avoid contributing sensitive information][sensitive-information].
               {% if category.icon %}
                 {% include uswds-icon.html icon=category.icon %}
               {% endif %}
-              <strong>
-                <a href="{{ category.url | prepend: site.baseurl }}" class="usa-link">{{ category.title }}</a>
+              <strong markdown="1">
+                [{{category.title}}]({% include article_url.txt article=category %})
               </strong>
             </span>
           </div>
@@ -45,7 +45,7 @@ to [avoid contributing sensitive information][sensitive-information].
 
 {% for category in site.categories %}
 
-### [{{ category.title }}]({{ category.url | prepend: site.baseurl }})
+### [{{category.title}}]({% include article_url.txt article=category %})
 
 {{ category.description}}