Downloads page: list the XTable artifact jar paths under each release

### Describe the request

On the [Downloads page](https://xtable.apache.org/releases/downloads) (`website/releases/downloads.mdx`), each release currently lists only:

- **Source Release** (`.src.tgz` with `.asc` / `.sha512`)
- **Release Notes**
- **Maven Search** link (to central.sonatype.com)

There is **no direct path/link to the XTable artifact (binary) jars**. Users who just want the built jars — especially the runnable bundled utilities jar — have to go through Maven search and navigate to find them.

### Proposal

Under **each release** on the Downloads page, add the path/links to the published XTable artifact jars (convenience binaries on Maven Central), e.g.:

- The Maven Central directory for the release, e.g.
`https://repo1.maven.org/maven2/org/apache/xtable/`
- The runnable bundled CLI jar, e.g.
`https://repo1.maven.org/maven2/org/apache/xtable/xtable-utilities_2.12//xtable-utilities_2.12--bundled.jar`
- (optionally) the other published artifacts: `xtable-api`, `xtable-core_2.12`, `xtable-hudi-support-utils`, etc.

Example for `0.3.0-incubating`:

```
### Release 0.3.0-incubating
* Source Release: ... (unchanged)
* Binary Artifacts: [Maven Central](https://repo1.maven.org/maven2/org/apache/xtable/) —
bundled CLI: [xtable-utilities_2.12-0.3.0-incubating-bundled.jar](https://repo1.maven.org/maven2/org/apache/xtable/xtable-utilities_2.12/0.3.0-incubating/xtable-utilities_2.12-0.3.0-incubating-bundled.jar)
* Release Notes: ... (unchanged)
* Maven Search: ... (unchanged)
```

### Notes

- The official ASF release remains the **source** release; the jars are convenience binaries published to Maven Central, which is acceptable to link from the Downloads page (as many ASF projects do).
- Keep the source-release link and signatures/checksums prominent; the artifact-jar links are an addition, not a replacement.
- This should be applied consistently to each release entry (0.2.0-incubating, 0.3.0-incubating, and future releases), and ideally documented in the release process so new releases include the artifact-jar links.

Edit website/releases/downloads.mdx, starting with the existing release entries for 0.2.0-incubating and 0.3.0-incubating. Add Maven Central directory and bundled CLI jar links while keeping the source-release, signature/checksum, release-notes, and Maven Search links. Done means every current release has consistent artifact links and the release-process guidance covers adding them for future releases.

Written by the indexing model from the issue text.