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.
Contributes to #2548
Follows up on a few comments in #4199
What I added/changed: