add a new third-party package in serializers.md

[khaledsukkar2]

Add DRF-Shapeless-Serializers to third-party packages

This PR adds DRF-Shapeless-Serializers to the documentation's third-party packages in the serializers section.

The package provides dynamic serializer configuration with:

• Runtime field selection/renaming
• Nested relationship control
• Conditional field logic
• Significant boilerplate reduction

[ulgens]

The package looks interesting, but isn't it a bit too early to recommend it?

[khaledsukkar2]

thank you,
I recommend this as it has successfully addressed many problems for my team and me.
so I want to share it with the community so everyone can benefit, and we can receive early feedback to address any potential issues.
Looking forward to seeing the positive impact this can have.
thank you again and I appreciate your time.