docs: update History of Redux with conference talk link

[Juneezee]

Hi @markerikson,

I came across your insightful comment on this Reddit post ^1 and wanted to thank you for sharing your knowledge! It was a valuable resource for me.

I believe adding a reference to Dan Abramov's conference talk in the documentation could further enrich the learning experience for users like myself who are interested in delving deeper.

Checklist

• [ ] Is there an existing issue for this PR?
  • link issue here
• [x] Have the files been linted and formatted?

What docs page is being added or updated?

• Section: Understanding Redux > History and Design
• Page: The History of Redux

For Updating Existing Content

What updates should be made to the page?

Add a link to Dan Abramov's demonstration for the Flux pattern at react-europe 2015.

Do these updates change any of the assumptions or target audience? If so, how do they change?

No.

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[markerikson]

Thanks!