Correct code file references in docs

The documentation was updated to add correct file references to the Python code snippets for Artist, Album, and Song models, and the function view. Previously, it was not clear where these code snippets should be placed.

Author: lipemorais

docs/index.md

@@ -131,6 +131,7 @@ We are going to create 3 models in the file models.py
 Let's start with the artist model.
 
 ```python
+# music.models.py
 class Artist(models.Model):
     name = models.CharField(max_length=100)
 
@@ -144,6 +145,7 @@ from django.db import models
 ```
 Now the album model
 ```python
+# music.models.py
 class Album(models.Model):
     title = models.CharField(max_length=100)
     artist = models.ForeignKey(Artist, on_delete=models.CASCADE)
@@ -156,6 +158,7 @@ class Album(models.Model):
 the last model will be the song model that will have relationship with artist and album.
 
 ```python
+# music.models.py
 class Song(models.Model):
     author = models.CharField(max_length=100)
     title = models.CharField(max_length=100)
@@ -197,6 +200,7 @@ And last but not least you need to create this
 you need to create the view. A function view in this case.
 
 ```python
+# music.views.py
 from django.http import HttpResponse