Task: link to genkit.dev in docs of all extensions that depend on genkit.

[cabljac]

Link to genkit.dev in extensions using Genkit

In the first upcoming version of any extension that depends on Genkit, we should link to [genkit.dev](http://genkit.dev/) via the readme and preinstall.

We need to say something like:
"This extension makes use of the Genkit SDK to (say what it does). For further information check out the genkit documentation at [genkit.dev](http://genkit.dev/)"

The extensions that use it are:

firebase/extensions:

• firestore-translate-text

GCP repo:

• firestore-genai-chatbot
• firestore-multimodal-genai
• firestore-vector-search

[cabljac]

We don't need to make releases for this yet, we can just include in the next release