Add docs for bird classification

docs/docs/configuration/bird_classification.md

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+---
+id: bird_classification
+title: Bird Classification
+---
+
+Bird classification identifies known bird using a quantized inat bird tensorflow model. When a known bird is recognized, its common name will be added as a `sub_label`. This information is included in the UI, filters, as well as in notifications.
+
+## Minimum System Requirements
+
+Bird classification runs a lightweight tflite model on the CPU, there are no significantly different system requirements than running Frigate itself.
+
+## Configuration
+
+Bird classification is disabled by default, it must be enabled in your config file before it can be used. Bird classification is a global configuration setting.
+
+```yaml
+classification:
+  bird:
+    enabled: true
+```
+
+## Advanced Configuration
+
+Fine-tune bird classification with these optional parameters:
+
+- `threshold`: Classification confidence score required to set the sub label on the object.
+  - Default: `0.9`.