docs: Add syntax highlighting to code examples

[Infamous003]

Added syntax highlighting to all the code examples in the source tutorial docs files for better readability.
Changes made to the following files:
reference: docs/tutorial

• 1-serialization.md
• 2-requests-and-responses.md
• 3-class-based-views.md
• 4-authentication-and-permissions.md
• 5-relationships-and-hyperlinked-apis.md
• 6-viewsets-and-routers.md
• quickstart.md

[Infamous003]

I've made the changes