Simplifying object detection colab (#240)

* Fixing object detection colab exemples that were not working anymore
* Adding a "What's next?" section at the end of the colab, pointing towards other examples.
* Simplifying open vocabulary object detection
* Removing the safety features (not blocking the prompts in the examples) 
* Grouping both parts with the castle drawing.
* Changing references to Gemini to Gemini API when it makes more sense
* changing dogs' picture for an open-sourced one and castle one for a Imagen generated one
* Removing interactive section to simplify
* Reordering examples.