feat: Cyclical relationship within a collection

[jaredcwhite]

I haven't specifically proved this, but I believe we have a gap in our current relations logic: what if I want a collection to "belongs_to" and also "has_many" itself? This would be useful in documentation where Page A could reference Page B and C and pages B and C could know about the back-reference from Page A.

Probably need to add a "belongs_to_with_references" or "belongs_to_self" or something to that nature. (It could be "has_and_belongs_to_many" but that sounds too much like a difference concept within Rails DB associations.)

I thought of this because I want to add "See Also" sections within our own documentation, and this feature would be the perfect fit. ☺️