Add Markup to Chapters

[jakarmy]

Markup on Playgrounds makes code more readable as it makes it clear what sections are real code, and what sections are just comments. It gives a much more elegant styling to the whole document. Some chapters are still using traditional comment syntax.
Example:
// This is a comment or /* This is a comment block */

The current status of each chapter regarding markup is:

• The Basics
• Basic Operators
• Strings and Characters
• Collection Types
• Control Flow
• Functions
• Closures
• Enumerations
• Classes & Structs
• Properties
• Methods
• Subscripts
• Inheritance
• Initialization
• Deinitialization
• ARC
• Optional Chaining
• Error Handling
• Type Casting
• Nested Types
• Extensions
• Protocols
• Generics
• Access Control

In order to write markup to chapters, please make sure to:

1. See other chapters with markup already in them as reference for writing markup in other chapters.
2. Include basic elements in each chapter such as:
   Common Header
   
   Headers
   
   Page Linkers
   [Previous](@previous) | [Next](@next)
   Paragraph Markers
   /*: This is a paragraph
   Other elements present in the Apple Markup Formatting Reference to add markup to the different chapters.
3. Link to other pages when possible, using the [Page Name](@page) syntax.
4. Follow the Apple Markup Formatting Reference to add markup to the different chapters.
5. Please, all changes to a specific chapter should be included in a separate branch, with its name specifying the Chapter's name and number: Branch name: markup_chapter_2_basic_operators.

This issue will be updated as chapters begin using markup.