THRIFT-5570: update java lib document about gradle usage

[jimexist]

update java lib document about gradle usage

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[ctubbsii]

Doc changes look good to me. I made only minor suggestions.

[jimexist]

had to rebase to pickup fix from #2585

[ctubbsii]

The doc changes are fine, but I made a comment about the .gitignore files. I don't want to include the new .gitignore file here. I'd prefer to reconcile the differences with the one in the lib/kotlin directory and the root directory, and just merge it into the one at the root for consistency. Can be done as part of this PR, or a separate one. But, I don't think the new file should be added here.