feat: enhance README files with code comments and variable declaration context

- Add a section on code comments in the main README, explaining their purpose and usage in TypeScript.
- Update the variables exercise README to clarify the differences between `var`, `let`, and `const`, emphasizing modern best practices.
- Modify the problem file to improve clarity on uncommenting code for demonstration purposes.

Author: kentcdodds

exercises/02.variables/01.problem.let-and-const/index.ts

@@ -20,7 +20,7 @@ const laptopPrice = 999.99
 // 🐨 Try uncommenting the line below - what happens?
 // TAX_RATE = 0.10
 
-// @ts-expect-error - 💣 remove this comment
-console.log('Cart subtotal:', cartTotal)
+// Uncomment the following lines to see the results
+// console.log('Cart subtotal:', cartTotal)
 // console.log('Tax:', cartTotal * TAX_RATE)
 // console.log('Final total:', finalTotal)

exercises/02.variables/README.mdx

@@ -44,3 +44,14 @@ arr.push(7) // Mutation - modifying the SAME array
 </callout-warning>
 
 Understanding this distinction helps you write predictable, bug-free code.
+
+<callout-info>
+	You might encounter `var` in older JavaScript code. It's the original way to
+	declare variables, but it has confusing scoping rules (function scope instead
+	of block scope) and allows redeclaration. Modern JavaScript introduced `let`
+	and `const` specifically to fix these issues. There's no reason to use `var`
+	in new code, which is why we focus exclusively on `let` and `const`.
+</callout-info>
+
+📜 For more details, see MDN's guide on [variable
+declarations](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Grammar_and_types#declarations).

exercises/README.mdx

@@ -36,3 +36,28 @@ By the end of this workshop, you'll be able to:
 - Understand how TypeScript helps catch errors before your code runs
 
 Let's build something solid! 🏗️
+
+<callout-info>
+	**Code comments** are notes you write in your code that explain what it does.
+</callout-info>
+
+Code comments start with `//` for single-line comments or `/* */` for multi-line
+comments. Comments are ignored by TypeScript—they're just for humans to read.
+You'll see comments throughout the exercises to help explain concepts. Feel
+free to add your own comments to help you understand the code! Here are some examples:
+
+```ts
+// This is a single-line comment
+
+/*
+This is a
+multi-line comment
+*/
+
+/*
+ * Sometimes you'll see multi-line comments
+ * with a star at the beginning and end.
+ * These extra stars are unnecessary, but
+ * look nice so people use them.
+ */
+```