docs (java-firestore): Broken redirects, samples and reference links in the README

The `java-firestore/README.md` contains multiple broken reference links.

**Broken Elements**

1. `Client Library Documentation`: Points to the [client library history](https://cloud.google.com/java/docs/reference/google-cloud-firestore/latest/history).
- **Expected:** Should point to `https://cloud.google.com/java/docs/reference/google-cloud-firestore` ( or the `/latest/overview` landing page).
- **Locations:** Found in the [first section of the README](https://github.com/googleapis/google-cloud-java/blob/main/java-firestore/README.md#google-cloud-firestore-client-for-java) and under the section called [About Cloud Firestore](https://github.com/googleapis/google-cloud-java/blob/main/java-firestore/README.md#about-cloud-firestore).

2. `Authentication`: Points to a [non-existent anchor tag](https://github.com/googleapis/google-cloud-java#authentication) in the repository root README.
- **Location:** Found under the [Authentication Section](https://github.com/googleapis/google-cloud-java/blob/main/java-firestore/README.md#authentication).

3. `Samples`: The most severely broken links are found here. The main link for [`samples/`](https://github.com/googleapis/google-cloud-java/tree/main/samples) points to a non-existent directory. Additionally, inside the table, all `source code` links point to non-existent destinations as well possibly because they are missing the `java-firestore/` subdirectory context.
- **Location:** Found under the [Samples section](https://github.com/googleapis/google-cloud-java/blob/main/java-firestore/README.md#samples).

4. `shared Troubleshooting document`: Points to a [non-existent destination](https://github.com/googleapis/google-cloud-java/blob/main/TROUBLESHOOTING.md) at the root level.
- **Location:** Found under the [Troubleshooting section](https://github.com/googleapis/google-cloud-java/blob/main/java-firestore/README.md#troubleshooting).