Add 184-elastic collisions challenge

[kfahn22]

This is still missing:

• index image
• description
• videoID
• link to Dipam's paper
• link to other paper mentioned

Date is currently set to today--will need to be updated.

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[dipamsen]

2d collisions article on vobarian.com: https://www.vobarian.com/collisions/2dcollisions2.pdf

My derivation/explanation of the collision formulae: https://dipamsen.github.io/notebook/collisions.pdf

Also, shouldn't the nature of code link point to https://natureofcode.com/? at least that's the link shown on the video multiple times. (No starch pre order page is accessible from the NoC home page, along with other ways to order)

[kfahn22]

@dipamsen Thanks for the links! I have added the link to the NOC website, but kept the link for preorder. @shiffman you can decide whether to delete.

[shiffman]

I've added a description, index image, and made other small changes. @dipamsen I think it works to have both the main URL and the no starch page as the links in the reference. I was hoping to have a way to order direct from natureofcode.com by now (I'm in touch with the warehouse/distributor on a way to integrate a shopify cart from noc.com itself) but I don't have all the pieces yet and I don't want to wait to release....

[shiffman]

The video is now members only I'll make public tomorrow Saturday July 13. Merging but welcome additional fixes and edits in new PRs!