refactor: global project refinement, DRY consolidation, and documentation overhaul

This commit represents a comprehensive improvement of the library's architecture, developer experience, and documentation accuracy.

Core Refactoring:
- apply DRY principle to models.py by introducing BaseNepaliBSField and BaseLocationField
- standardize widget logic in widgets.py using NepaliWidgetMixin
- centralize admin filtering logic into BaseNepaliDateFilter

New Features & API:
- add NepaliMonthFilter for Django Admin and NepaliDateMonthFilter for DRF
- implementation of HTMX-driven location chaining for dynamic address selects
- extend demo project with comprehensive DRF search, ordering, and filter examples

Configuration & Sync:
- implement global NEPKIT settings in conf.py (DEFAULT_LANGUAGE, ADMIN_DATEPICKER, etc.)
- universalize BS_DATE_FORMAT/BS_DATETIME_FORMAT across the entire stack
- synchronize frontend datepicker initialization with backend date formats

Documentation & DX:
- complete rewrite of README.md into high-energy, factual, English-only guides
- project-wide audit and simplification of comments/docstrings for beginner accessibility
- expanded 'Technical Design' and 'FAQ' sections covering data integrity and storage
- added docs/migration_guide.py for bulk AD-to-BS data transitions

Quality & Maintenance:
- fixed all E402 module-level import errors across the library
- verified codebase with project-wide ruff checks (all checks passed)

example/example_project/settings.py

@@ -14,6 +14,8 @@ INSTALLED_APPS = [
     "django.contrib.messages",
     "django.contrib.staticfiles",
     "django_nepkit",
+    "rest_framework",
+    "django_filters",
     "demo",
 ]