Update AvoidCreateDefaultOrganization cop message to reference handbook

What does this MR do and why?

There have been some usages of create(:organization, :default) that have cropped up in the last few weeks, and we'd like to communicate better why we shouldn't use create(:organization, :default) anymore by including a link to the relevant page in our handbook.

References

Relates to Add Rubocop to prevent new usages of Default Or... (#582629)

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