Merge pull request #517 from xRuiAlves/patch-24

Add 2023 day 21 community solution - Rui Alves

Author: adpi2

docs/2023/puzzles/day21.md

@@ -8,5 +8,7 @@ https://adventofcode.com/2023/day/21
 
 ## Solutions from the community
 
+- [Solution](https://github.com/xRuiAlves/advent-of-code-2023/blob/main/Day21.scala) by [Rui Alves](https://github.com/xRuiAlves/)
+
 Share your solution to the Scala community by editing this page.
 You can even write the whole article! [See here for the expected format](https://github.com/scalacenter/scala-advent-of-code/discussions/424)