Update serialization examples from XML to JSON

[Saksham Jain]

Most of the serialization documentation currently demonstrates usage with the XML serializer. This made sense historically, when XML was the common format for data exchange. However, JSON has since become the de facto standard for serialization in modern Django projects.

To reflect current best practices and developer expectations, the documentation should be updated to show examples using json instead of xml. This change would make the examples more relevant and immediately usable, while still acknowledging that XML remains supported.

[Sarah Boyce <42296566+sarahboyce@…>]

In 762d3be:

Fixed #36581 -- Updated serialization examples from XML to JSON.

[Sarah Boyce <42296566+sarahboyce@…>]

In 16bc8de:

[6.0.x] Fixed #36581 -- Updated serialization examples from XML to JSON.

Backport of 762d3be8c559b0abf415be8d6117f04fb6347983 from main.

[Sarah Boyce <42296566+sarahboyce@…>]

In 7554c54e:

[5.2.x] Fixed #36581 -- Updated serialization examples from XML to JSON.

Backport of 762d3be8c559b0abf415be8d6117f04fb6347983 from main.