[DOC] Improve "Getting started" flow with Java docs

[bryanhuhta]

After pairing for some time with an SE, I learned that our Java docs weren't helpful at providing an opinionated quickstart guide. The observation was that the Java Agent is the easiest and least invasive way to test out profiling on a Java app, so we should highlight that strategy.

I tend to agree here. Unless there's specific technical reasons as to why we advise against the Java Agent, we should revamp the docs to provide a quickstart guide which focuses on the Java Agent approach.