Added Remix IDE to the Playgrounds file (#5598)

* Added Remix IDE to the Playgrounds file

A powerful Browser IDE for playing around with Solidity smart contracts!

* Update more/free-programming-playgrounds.md
Co-authored-by: NDavid Ordás <3125580+davorpa@users.noreply.github.com>

* whitespace
Co-authored-by: NDavid Ordás <3125580+davorpa@users.noreply.github.com>

* spaces
Co-authored-by: NDavid Ordás <3125580+davorpa@users.noreply.github.com>
Co-authored-by: NEric Hellman <eric@hellman.net>

more/free-programming-playgrounds.md

@@ -33,6 +33,7 @@
 * [Rust](#rust)
 * [RxJS](#rxjs)
 * [Scala](#scala)
+* [Solidity](#solidity)
 * [SQL](#sql)
 * [Svelte](#svelte)
 * [Swift](#swift)
@@ -246,6 +247,11 @@
 * [Scastie](https://scastie.scala-lang.org)
 
 
+### Solidity
+
+* [Remix IDE](https://remix.ethereum.org)
+
+
 ### SQL
 
 * [Extends Class](https://extendsclass.com/sqlite-browser.html)