Add variables section in docs

docs/SUMMARY.md

@@ -3,3 +3,5 @@
 - [Introduction](./introduction/SUMMARY.md)
   - [Installation](./introduction/installation.md)
   - [Hello World!](./introduction/hello-world.md)
+- [Common language concepts](./concepts/SUMMARY.md)
+  - [Variables](./concepts/variables.md)

docs/concepts/SUMMARY.md

@@ -0,0 +1,5 @@
+# Common language concepts
+
+This chapter covers concepts that appear in almost every programming language and how they work in Rust. Many programming languages have much in common at their core.
+
+Specifically, you’ll learn about variables, basic types, functions, comments, and control flow. These foundations will be in every Sabre program, and learning them early will give you a strong core to start from.

docs/concepts/variables.md

@@ -0,0 +1,22 @@
+# Variables
+
+If you are familiar with some other programming language, the way Sabre handles variables won't surprise you.
+
+To declare a variable, the `let` keyword is used.
+
+```
+// variables.sb
+fn main() {
+    let x = 10
+    let y = 5
+    return x + y
+}
+```
+
+Run this code using the sabre CLI:
+
+```
+$ sabre build variables.sb -o variables.js
+$ node variables.js
+15
+```

examples/playground.sb

@@ -1,8 +1,4 @@
 fn main() {
-    let x = true && false
-    let y = false && true || true
-    let z = x && true
-    while true && x {
-        return x
-    }
-}
+    let x = 10
+    return x + 10
+}

src/parser/tests.rs

@@ -89,6 +89,20 @@ fn test_parse_variable_declaration() {
     assert!(tree.is_ok())
 }
 
+#[test]
+fn test_parse_variable_declaration_added() {
+    let raw = "
+    fn main() {
+        let x = 10
+        let y = 5
+        return x + y
+    }
+    ";
+    let tokens = tokenize(raw);
+    let tree = parse(tokens, Some(raw.to_string()));
+    assert!(tree.is_ok())
+}
+
 #[test]
 fn test_parse_function_with_args() {
     let raw = "