docs: How to download the artifacts of opendal-java

[Xuanwo]

Download the artifacts from https://repository.apache.org/content/repositories/orgapacheopendal-${maven_artifact_number}/.

We need more detailed steps here

[Xuanwo]

cc @tisonkun, would you like to take a look?

[tisonkun]

No. This is the release step and exactly the URL. For end-user usage, refer to the README.

[tisonkun]

I guess you don't read this section: https://opendal.apache.org/docs/contributing/release#release-maven-artifacts

[Xuanwo]

I guess you don't read this section: https://opendal.apache.org/docs/contributing/release#release-maven-artifacts

Oh, my problem happens at verify stage: I don't know how to verify the artifacts that already updated to the repo, for example, https://repository.apache.org/content/repositories/orgapacheopendal-1014/.

[tisonkun]

Now it's avialable - https://repository.apache.org/content/repositories/orgapacheopendal-1014/

We should close the candidate first. I'll prepare a update for the release process.