Mention `set_preview_images` config in Bard/Replicator docs

[duncanmcclean]

Developers need to add the set_preview_images key to their assets config (it's there by default for new sites), otherwise the preview image option won't show.

We should mention it in the docs.

https://github.com/statamic/cms/blame/7c50e687ff5f94cd91ace6a9ead418eaeb2754c1/config/assets.php#L251-L265