While I do not know what software the Atlassian Community runs on I want to propose an alternative:
Confluence is great for collaborating and chatting with your customers. With apps like Questions for Confluence you are able to archive Q&A.
If you are on a low budget you could host a OpenSource forum software instead. There are several OpenSource solutions like phpBB or Discourse. Look them up. :)
But even if you are on a low budget Confluence Server could still be usable. If you go with a 10 User license (including possibly needed apps) and allow public access for example.
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