Add docs for classification models (#20188)

docs/docs/configuration/custom_classification/object_classification.md

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+---
+id: object_classification
+title: Object Classification
+---
+
+Object classification allows you to train a custom MobileNetV2 classification model to run on tracked objects (persons, cars, animals, etc.) to identify a finer category or attribute for that object.
+
+## Minimum System Requirements
+
+Object classification models are lightweight and run very fast on CPU. Inference should be usable on virtually any machine that can run Frigate.
+
+Training the model does briefly use a high amount of system resources for about 1–3 minutes per training run. On lower-power devices, training may take longer.
+When running the `-tensorrt` image, Nvidia GPUs will automatically be used to accelerate training.
+
+### Sub label vs Attribute
+
+- **Sub label**:
+
+  - Applied to the object’s `sub_label` field.
+  - Ideal for a single, more specific identity or type.
+  - Example: `cat` → `Leo`, `Charlie`, `None`.
+
+- **Attribute**:
+  - Added as metadata to the object (visible in /events): `<model_name>: <predicted_value>`.
+  - Ideal when multiple attributes can coexist independently.
+  - Example: Detecting if a `person` in a construction yard is wearing a helmet or not.
+
+## Example use cases
+
+### Sub label
+
+- **Known pet vs unknown**: For `dog` objects, set sub label to your pet’s name (e.g., `buddy`) or `none` for others.
+- **Mail truck vs normal car**: For `car`, classify as `mail_truck` vs `car` to filter important arrivals.
+- **Delivery vs non-delivery person**: For `person`, classify `delivery` vs `visitor` based on uniform/props.
+
+### Attributes
+
+- **Backpack**: For `person`, add attribute `backpack: yes/no`.
+- **Helmet**: For `person` (worksite), add `helmet: yes/no`.
+- **Leash**: For `dog`, add `leash: yes/no` (useful for park or yard rules).
+- **Ladder rack**: For `truck`, add `ladder_rack: yes/no` to flag service vehicles.
+
+## Configuration
+
+Object classification is configured as a custom classification model. Each model has its own name and settings. You must list which object labels should be classified.
+
+```yaml
+classification:
+  custom:
+    dog:
+      threshold: 0.8
+      object_config:
+        objects: [dog] # object labels to classify
+        classification_type: sub_label # or: attribute
+```
+
+## Training the model
+
+Creating and training the model is done within the Frigate UI using the `Classification` page.
+
+### Getting Started
+
+When choosing which objects to classify, start with a small number of visually distinct classes and ensure your training samples match camera viewpoints and distances typical for those objects.
+
+// TODO add this section once UI is implemented. Explain process of selecting objects and curating training examples.
+
+### Improving the Model
+
+- **Problem framing**: Keep classes visually distinct and relevant to the chosen object types.
+- **Data collection**: Use the model’s Train tab to gather balanced examples across times of day, weather, and distances.
+- **Preprocessing**: Ensure examples reflect object crops similar to Frigate’s boxes; keep the subject centered.
+- **Labels**: Keep label names short and consistent; include a `none` class if you plan to ignore uncertain predictions for sub labels.
+- **Threshold**: Tune `threshold` per model to reduce false assignments. Start at `0.8` and adjust based on validation.