Add module-level docstrings for each app

InvenTree/InvenTree/__init__.py

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+"""
+The InvenTree module provides high-level management and functionality.
+
+It provides a number of helper functions and generic classes which are used by InvenTree apps.
+"""

InvenTree/build/__init__.py

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+"""
+The Build module is responsible for managing "Build" transactions.
+
+A Build consumes parts from stock to create new parts
+"""

InvenTree/company/__init__.py

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+"""
+The Company module is responsible for managing Company interactions.
+
+A company can be either (or both):
+
+- Supplier
+- Customer
+"""

InvenTree/part/__init__.py

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+"""
+The Part module is responsible for Part management.
+
+It includes models for:
+
+- PartCategory
+- Part
+- SupplierPart
+- BomItem
+"""

InvenTree/stock/__init__.py

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+"""
+The Stock module is responsible for Stock management.
+
+It includes models for:
+
+- StockLocation
+- StockItem
+- StockItemTracking
+"""