Auto merge of #5911 - arbonap:pa-setup-docs, r=segiddins
Auto merge of #5911 - arbonap:pa-setup-docs, r=segiddins

Explain RUBYOPT in Debugging section of Setup docs

Thanks so much for the contribution!
To make reviewing this PR a bit easier, please fill out answers to the following questions.

### What was the end-user problem that led to this PR?

The problem was...
- I was looking through `SETUP.md` and I didn't understand why I had to set the `RUBYOPT` env variable to `-rpry dbundle`. I was wondering why I couldn't just normally use `require 'pry'; binding.pry` to debug Bundler's codebase.

### What was your diagnosis of the problem?

My diagnosis was...
- To add more explanations to the Setup documentation.

### What is your fix for the problem, implemented in this PR?

My fix...
- I asked @indirect and @jules2689 all about `RUBYOPT`, and why we have to set `RUBYOPT` to require pry and require this path to Bundler. I used their wonderful explanations and tried synthesizing their knowledge in a way that made sense to those who aren't familiar with `RUBYOPT`. I also included background context as to why Bundler chooses to do this.

### Why did you choose this fix out of the possible options?

I chose this fix because...
- Having background context of how and why Bundler combines `RUBYOPT` with `dbundle` in the Setup docs will help new contributors ramp-up.
- I also think having this knowledge will (hopefully) help new contributors not put in `require 'pry'; binding.pry` like they may be used to in other Ruby projects. ✨

Let me know what y'all think of this first draft 👌🏼. I'm happy to incorporate changes.

dilumnavanjana/bundler,bundler/bundler,mvz/bundler,bundler/bundler,mvz/bundler,dilumnavanjana/bundler