Add warnings about spam filters, etc

Hi, it would be good it the righthand tips window warns about spam filters. My was filtered for having too many links.

For example,

Welcome to Hugging Face Forums — thanks for starting a new conversation!

Warning: Posts are subject to automatic spam filter detection (e.g., more than one link. For more details, see TODO.

I did check the new-posting guide and another link provided during the editing; however, I saw nothing as explicit as the above. I also did search for related content with no luck.

By the way, it would also be good to warn that the preview is hidden under the warning.