Add t-shirt design links to design files (#1166)

Adds a link to the Figma Community file for the t-shirt designs we have discussed in the community. Anyone is free to contribute more designs, or print and wear them as they desire and bring down the house with their fresh fashion vibes.

Author: GBKS

assets/images/guide/resources/design-files/shirts.jpg

@@ -64,6 +64,12 @@ files:
     retina: /assets/images/guide/resources/design-files/[email protected]
     width: 400
     height: 210
+  - caption: <a href="https://www.figma.com/community/file/1499681205477242413/bitcoin-design-shirts">Bitcoin Design T-Shirts</a>
+    alt: Close-up of a t-shirt with Bitcoin Design Community and Foundation logos, and the text 'Make magic internet money feel magical'
+    image: /assets/images/guide/resources/design-files/shirts.jpg
+    retina: /assets/images/guide/resources/design-files/[email protected]
+    width: 400
+    height: 210
 ---
 
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