[docs] Added docs for vertx pg client for Java

[priyanshi-yb]

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This PR adds the docs for Vertx PG Client in build-app for Java and mentions as supported client drivers in reference.

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[vkulichenko]

@priyanshi-yb Should we add these changes to other versions as well, in addition to preview?

[priyanshi-yb]

@vkulichenko We generally add the docs in the preview version only. I guess @polarweasel can tell us better about the other versions for the docs to be added.

[polarweasel]

I would say the build-apps stuff at least needs only to be in /preview. The changes in the driver file, though, should be in every version to which they're applicable.