[Docs] Document API stability

[mbleigh]

We are adding import { genkit} from "genkit/beta"; to enable the ability to opt-in to unstable features of Genkit. We should clearly document the differences in expectations between the two stability levels:

• Stable: No changes except for major version. Recommend ^1.0.0 dependency.
• Beta: Breaking changes in minor versions. Recommend ~1.X.0 dependency.