Update java sampling documentation page

[jaydeluca]

Contributes to #2548

Follows up on a few comments in #4199

What I added/changed:

• Documented the ability to configure with environment variables in a top-level section in this guide, added link to configuration page with all options.
• Added a note above the custom code section about other ways of sampling (agent, spring starter etc)
• Emphasized that most the defaults suffice for most users, and that most users don't need to customize the sampler.
• Removed the production guidance at the bottom of the page - there seemed to be some pushback in the original PR and I also didn't find the recommendation to be very clear. I can add back in or update if needed.
• Added some more context about the different sampler types
• Tried to follow similar layouts and content as other languages, but there isn't a whole lot of consistency.

[theletterf]

Thanks! Just a couple of suggestions.

+CC @open-telemetry/java-maintainers Please have a look.

[cartermp]

@open-telemetry/java-approvers please take a look!

[svrnm]

@open-telemetry/java-approvers please take a look!

[svrnm]

@jaydeluca thank you for your continued contributions!