Add Signature ✍️ dataset in docs (#12977)

Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

ultralytics/cfg/datasets/signature.yaml

@@ -0,0 +1,20 @@
+# Ultralytics YOLO 🚀, AGPL-3.0 license
+# Signature dataset by Ultralytics
+# Documentation: https://docs.ultralytics.com/datasets/detect/signature/
+# Example usage: yolo train data=signature.yaml
+# parent
+# ├── ultralytics
+# └── datasets
+#     └── signature  ← downloads here (11.2 MB)
+
+# Train/val/test sets as 1) dir: path/to/imgs, 2) file: path/to/imgs.txt, or 3) list: [path/to/imgs1, path/to/imgs2, ..]
+path: ../datasets/signature # dataset root dir
+train: train/images # train images (relative to 'path') 143 images
+val: valid/images # val images (relative to 'path') 35 images
+
+# Classes
+names:
+  0: signature
+
+# Download script/URL (optional)
+download: https://ultralytics.com/assets/signature.zip