Add "Retired" product class category

The subcategories of the "Official Hardware" category are to be organized under a subcategory for each of the product
classes.

However, this classification is not done for hardware products which are no longer manufactured. A "Retired" category
will be used as the container for the existing categories for retired products.

Author: per1234

content/categories/official-hardware/retired/README.md

@@ -0,0 +1,11 @@
+# Retired
+
+## Permissions
+
+| Group    | See | Reply | Create |
+| -------- | --- | ----- | ------ |
+| everyone | ✓   | ✓     | ✓      |
+
+## Published At
+
+https://forum.arduino.cc/c/official-hardware/retired/200

content/categories/official-hardware/retired/_pins.md

@@ -0,0 +1,2 @@
+- https://forum.arduino.cc/t/about-the-retired-category/1335295
+- https://forum.arduino.cc/t/how-to-get-the-best-out-of-this-forum/1335296

content/categories/official-hardware/retired/_topics/about-the-retired-category/1.md

@@ -0,0 +1 @@
+Arduino products that have been retired

content/categories/official-hardware/retired/_topics/about-the-retired-category/README.md

@@ -0,0 +1,5 @@
+# About the Retired category
+
+## Published At
+
+https://forum.arduino.cc/t/about-the-retired-category/1335295

content/categories/official-hardware/retired/_topics/how-to-get-the-best-out-of-this-forum

@@ -0,0 +1 @@
+../../../../_topics/how-to-get-the-best-out-of-this-forum

content/categories/order.md

@@ -86,6 +86,7 @@
     - Nicla Vision
     - Nicla Voice
   - Opta
+  - Retired
   - Yún Shield
 - Other Hardware
   - 3rd Party Boards