[docs]: add Pika H3 Flex to the cookbook

Three source recipes (T2VA, keyframes, Ref2VA) on one model page under
MiniMax H3, with knobs for task, resolution tier, aspect bucket,
duration, NVFP4 decoder and text encoder, offload mode, and GPU count.
The page uses the official Pika-Labs Hugging Face avatar.

docs/generate_examples.py

@@ -39,6 +39,7 @@ COOKBOOK_FAMILIES = {
     "qwen_image21",
     "sd35",
     "minimax_h3",
+    "pika_h3_flex",
     "stable_audio",
     "mmaudio",
     "matrixgame",