feat: Add create_document_reference_content helper

[GnautSpace]

Description

Created a helper function create_document_reference_content for creating DocumentReferenceContent objects

Related Issue

Resolves #138

Changes Made

• add create_document_reference_content() helper following existing patterns
• supports text/plain, text/html, application/pdf and other MIME types
• includes comprehensive docstring with usage examples
• add unit tests for data, URL, language, and kwargs scenarios
• export function in healthchain/fhir/init.py

Testing

• all necessary imports are present (Coding is imported)
• create_document_reference_content is imported from helpers
• all 4 test functions are present:
  - test_create_document_reference_content_with_data()
  - test_create_document_reference_content_with_url()
  - test_create_document_reference_content_custom_language()
  - test_create_document_reference_content_with_kwargs()
• tests follow existing patterns and are comprehensive

Checklist

• [x ] I have read the contributing guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• [x ] My changes generate no new warnings
• [x ] I have added tests that prove my fix is effective or that my feature works
• [x ] New and existing unit tests pass locally with my changes

Additional Notes