Publish the OkHttp interceptor module (sdk-okhttp) to Maven Central alongside android-sdk 3.0.0

## Summary

Since `3.0.0`, the base SDK is published to **Maven Central** as `com.authgear:android-sdk:3.0.0`, but the **OkHttp interceptor module (`sdk-okhttp`) is not published there**. There's currently no Maven Central coordinate a developer can add to get `AuthgearInterceptor` alongside the Maven Central base SDK.

## Evidence

Base SDK is on Maven Central; the interceptor is not:

```
com.authgear:android-sdk -> HTTP 200 (repo1.maven.org)
com.authgear:android-sdk-okhttp -> HTTP 404
com.authgear:authgear-sdk-android-okhttp -> HTTP 404
```

In the source at tag `3.0.0`:

- `sdk/build.gradle.kts` uses the Vanniktech plugin with `publishToMavenCentral()` and `coordinates("com.authgear", "android-sdk", …)` → publishes to Central. ✅
- `sdk-okhttp/build.gradle.kts` uses only `id("maven-publish")` with `artifactId = "authgear-sdk-android-okhttp"` (no `publishToMavenCentral()` / Vanniktech config) → **not** published to Maven Central.

So the interceptor (`com.oursky.authgear.okhttp.AuthgearInterceptor`) has no Central release to pair with `android-sdk:3.0.0`.

## Impact

- The docs page [OkHttp interceptor extension](https://docs.authgear.com/get-started/native-mobile-app/android/okhttp-interceptor-extension) can't give a working Maven Central dependency line. (I updated the main Android SDK page to Maven Central in authgear/docs#47, but intentionally left the OkHttp page alone because there's no coordinate to document yet.)
- Developers who follow the (now Maven Central) base SDK setup and then want the OkHttp interceptor have no clean, consistent way to add it.

## Suggested fix

Publish `sdk-okhttp` to Maven Central with the same tooling/versioning as the base module — e.g. `com.authgear:android-sdk-okhttp:3.0.0` (Vanniktech `publishToMavenCentral()` + matching `coordinates(...)` and version), so the two artifacts stay in lockstep. Then the docs page can document the dependency.

## Related (minor, separate)

The OkHttp doc's code sample mixes Java/Kotlin syntax (`new OkHttpClient.Builder()....addInterceptor(AuthgearInterceptor(authgear))`) and reads as incomplete — worth a small docs cleanup once the coordinate exists.

---

Context: found while modernizing the [authgear-example-android](https://github.com/authgear/authgear-example-android) sample (PR #5) and reconciling the Android docs.

Start with sdk-okhttp/build.gradle.kts and compare its maven-publish setup with sdk/build.gradle.kts, which uses the Vanniktech plugin and publishes com.authgear:android-sdk. Configure the interceptor module to publish as com.authgear:android-sdk-okhttp with the matching 3.0.0 version, then verify the Maven Central publication and coordinate alongside the base SDK.

Written by the indexing model from the issue text.