Merge pull request #358 from per1234/document-staging-site-in-test-category

Add note re: availability of "staging site" forum to "About the TEST category" post

Author: per1234

content/categories/staff/test/_topics/about-the-test-category/1.md

@@ -1,3 +1,11 @@
 A sandbox in which Arduino Forum staff can experiment freely. Expect strange and nonsensical content.
 
 DO NOT post anything of value or that you want others to pay attention to in this category.
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+---
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+**ⓘ** An isolated "sandbox" forum instance is also available. The "staging site" forum can be used for more involved testing and experimentation without any risk of affecting the production forum users.
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+Instructions for accessing the "staging site" forum are [**here**](https://forum.arduino.cc/t/staging-site-sandbox-forum-instance/1112538).
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+---