docs: Add comprehensive fixture support documentation

[JohananOppongAmoateng]

Add Comprehensive Fixture Support Documentation
Users reported difficulty using Django's loaddata command with polymorphic models. While the documentation mentioned using --natural-primary and --natural-foreign flags with dumpdata , there was no guidance on:

How to properly create fixtures for polymorphic models
How to load fixtures with loaddata
What to do if polymorphic_ctype
references are incorrect after loading
Solution
This PR adds comprehensive documentationfor fixture support with polymorphic models.

advanced.rst
Added new section "Working with Fixtures" with:

Creating fixtures with dumpdata --natural-primary --natural-foreign
Loading fixtures with loaddata
Using reset_polymorphic_ctype() utility to fix incorrect references
Example fixture JSON format with natural keys
Data migration example for fixing imported data

quickstart.rst
Enhanced the existing note about fixtures with:

Closes #513

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.44%. Comparing base (0d30c86) to head (905600b).

Additional details and impacted files

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  Coverage   83.44%   83.44%           
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  Files          26       26           
  Lines        1649     1649           
  Branches      264      264           
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  Hits         1376     1376           
  Misses        207      207           
  Partials       66       66           

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