Add documentation for PairedFieldDescriptor implementation

Author: Kirandawadi

sde_collections/utils/README_PAIRED_FIELD_DESCRIPTOR.md

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+# Paired Field Descriptor System
+
+## Overview
+
+The Paired Field Descriptor is a Django model descriptor designed to manage fields with both manual and machine learning (ML) generated variants. This system provides a flexible approach to handling metadata fields, with a focus on tag management and priority handling.
+
+## Core Concepts
+
+### Field Pairing Mechanism
+The descriptor automatically creates two associated fields for each defined descriptor:
+- **Manual Field**: Manually entered or curated metadata
+- **ML Field**: Machine learning generated metadata
+
+### Key Characteristics
+- Manual field takes precedence over ML field
+- Flexible field type support
+- Handles empty arrays and None values
+- Requires explicit setting of ML fields
+
+## Implementation
+
+### Creating a Paired Field Descriptor
+
+```python
+tdamm_tag = PairedFieldDescriptor(
+    field_name="tdamm_tag",
+    field_type=ArrayField(models.CharField(max_length=255, choices=TDAMMTags.choices), blank=True, null=True),
+    verbose_name="TDAMM Tags",
+)
+```
+
+#### Parameters
+- `field_name`: Base name for the descriptor
+- `field_type`: Django field type (supports various field types)
+- `verbose_name`: Optional human-readable name
+
+### Field Naming Convention
+When you define a descriptor, two additional fields are automatically created:
+- `{field_name}_manual`: For manually entered values
+- `{field_name}_ml`: For machine learning generated values
+
+## Characteristics
+
+### Field Priority
+1. Manual field always takes precedence
+2. ML field serves as a fallback
+3. Empty manual fields or None values defer to ML field
+
+### Field Retrieval
+```python
+# Retrieval automatically prioritizes manual field
+tags = url.tdamm_tag  # Returns manual tags if exist, otherwise ML tags
+```
+
+### Field Setting
+```python
+# Sets only the manual field
+url.tdamm_tag = ["MMA_M_EM", "MMA_M_G"]
+
+# ML field must be set explicitly
+url.tdamm_tag_ml = ["MMA_O_BH"]
+```
+
+### Field Deletion
+```python
+# Deletes both manual and ML fields
+del url.tdamm_tag
+```
+
+### Data Preservation
+- Paired fields maintain their state during:
+  - Dump to Delta migration
+  - Delta to Curated promotion
+- Manual entries take precedence in all migration stages
+
+## Serializer Integration
+
+Here's the way to configure the serializer to retrieve the paired field, seamlessly extracting either manual or ML tags based on the descriptor's priority rules.
+```python
+class DeltaUrlSerializer(serializers.ModelSerializer):
+    tdamm_tag = serializers.SerializerMethodField()
+
+    class Meta:
+        model = DeltaUrl
+        fields = ("url", "tdamm_tag")
+
+    def get_tdamm_tag(self, obj):
+        tags = obj.tdamm_tag
+        return tags if tags is not None else []
+```