fix hello world to use felts + add felts concept

concepts/felts/.meta/config.json

@@ -0,0 +1,7 @@
+{
+    "blurb": "<todo>",
+    "authors": [
+        "misicnenad"
+    ],
+    "contributors": []
+}

concepts/felts/about.md

@@ -0,0 +1 @@
+# Felt Type

concepts/felts/introduction.md

@@ -0,0 +1 @@
+# Introduction

concepts/felts/links.json

@@ -0,0 +1,10 @@
+[
+    {
+        "url": "https://book.cairo-lang.org/ch02-02-data-types.html#felt-type",
+        "description": "Felt type in the Cairo book"
+    },
+    {
+        "url": "https://starknet-by-example.voyager.online/getting-started/cairo_cheatsheet/felt.html",
+        "description": "Starknet by Example section on felts"
+    }
+]

config.json

@@ -40,7 +40,8 @@
         "name": "Hello World",
         "uuid": "1a0e23d9-e8f9-493a-af46-2be040173b64",
         "practices": [
-          "strings"
+          "strings",
+          "felts"
         ],
         "prerequisites": [],
         "difficulty": 1,
@@ -53,7 +54,8 @@
         "name": "Leap",
         "uuid": "9b586e17-928c-42bf-95ed-67539daea9a8",
         "practices": [
-          "booleans"
+          "booleans",
+          "integers"
         ],
         "prerequisites": [],
         "difficulty": 1,

exercises/practice/hello-world/src/lib.cairo

@@ -1,6 +1,6 @@
-// ByteArray is a string that's not limited to 31 characters, you'll learn more about this later
-fn hello() -> ByteArray {
-    "Goodbye, Mars!"
+// felt252, aka "field type", represents both a specific type of integer, and a string type that's limited to 31 characters
+fn hello() -> felt252 {
+    'Goodbye, Mars!'
 }
 
 #[cfg(test)]

exercises/practice/hello-world/src/tests.cairo

@@ -1,4 +1,4 @@
 #[test]
 fn say_hi() {
-    assert_eq!(hello_world::hello(), "Hello, World!");
+    assert_eq!(hello_world::hello(), 'Hello, World!');
 }