Reorganize predefined recognizers into logical subfolders

Move country-specific recognizers to country_specific/{country}/ folders
Move generic recognizers to generic/ folder
Move NER recognizers to ner/ folder
Maintain full backward compatibility for all existing imports
Update test imports to use new file locations
Add comprehensive documentation for new structure

Fixes #1638

Change Description

This PR reorganizes the predefined recognizers in presidio-analyzer into logical subfolders to improve code maintainability and make it easier for contributors to add new recognizers.

What Changed:

Directory Structure:

country_specific/ - Organizes recognizers by country (9 countries: US, UK, India, Italy, Australia, Spain, Finland, Poland, Singapore)
generic/ - Contains globally applicable recognizers (Credit Card, Crypto, Date, Email, IBAN, IP, Medical License, Phone, URL)
ner/ - Contains Named Entity Recognition based recognizers (SpaCy, Stanza, Transformers, GLiNER, Azure AI Language)

Files Moved:

28 country-specific recognizers moved to appropriate country folders
10 generic recognizers moved to generic/ folder
5 NER recognizers moved to ner/ folder

Backward Compatibility:

All existing imports continue to work unchanged (e.g., from presidio_analyzer.predefined_recognizers import CreditCardRecognizer)
Main __init__.py updated to import from new locations and re-export all classes
No breaking changes for existing users

Documentation:

Added comprehensive README.md explaining new structure and contribution guidelines
Updated CHANGELOG.md under Unreleased section as required
Added __init__.py files to all new directories

Test Updates:

Fixed 3 test files that had direct imports to recognizer files
All tests now use correct import paths for new structure

Benefits:

Better Organization: Clear separation makes codebase more maintainable
Easier Contributions: Contributors can easily find where to add new recognizers
Scalable: Simple to add new countries or recognizer types
Well Documented: Clear guidelines for future development

Issue reference

This PR fixes issue #1638

Checklist

[x] I have reviewed the contribution guidelines
[ ] I have signed the CLA (if required)
[x] My code includes unit tests (updated existing tests affected by reorganization)
[ ] All unit tests and lint checks pass locally
[x] My PR contains documentation updates / additions if required