docs(js): Adds an API Stability guide.

docs/api-stability.md

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+# API Stability Channels
+
+As of version 1.0, Genkit is considered **Generally Available (GA)** and ready
+for production use. Genkit follows [semantic versioning](https://semver.org/)
+with breaking changes to the stable API happening only on major version
+releases.
+
+To gather feedback on potential new APIs and bring new features out quickly,
+Genkit offers a **Beta** entrypoint that includes APIs that have not yet
+been declared stable. The beta channel may include breaking changes on
+*minor* version releases.
+
+## Using the Stable Channel
+
+To use the stable channel of Genkit, import from the standard `"genkit"`
+`"genkit"` entrypoint:
+
+```ts
+import { genkit, z } from "genkit";
+
+const ai = genkit({plugins: [...]});
+console.log(ai.apiStability); // "stable"
+```
+
+When you are using the stable channel, we recommend using the standard `^X.Y.Z`
+dependency string in your `package.json`. This is the default that is used when
+you run `npm install genkit`.
+
+## Using the Beta Channel
+
+To use the beta channel of Genkit, import from the `"genkit/beta"` entrypoint:
+
+```ts
+import { genkit, z } from "genkit/beta";
+
+const ai = genkit({plugins: [...]});
+console.log(ai.apiStability); // "beta"
+
+// now beta features are available
+```
+
+When you are using the beta channel, we recommend using the `~X.Y.Z` dependency
+string in your `package.json`. The `~` will allow new patch versions but will
+not automatically upgrade to new minor versions which may have breaking changes
+for beta features. You can modify your existing dependency string by changing
+`^` to `~` if you begin using beta features of Genkit.
+
+### Current Features in Beta
+
+- **[Chat/Sessions](chat):** a first-class conversational `ai.chat()` feature
+  along with persistent sessions that store both conversation history and an
+  arbitrary state object.
+- **[Interrupts](interrupts):** special tools that can pause generation for
+  human-in-the-loop feedback, out-of-band processing, and more.