[docs] Reorganize Nix mobile setup docs

Summary:
I think it makes more sense this way for two reasons:

1. This part of nix_dev_env.md effectively serves as a table of contents for nix_mobile_setup.md, but the structure of the two don't currently align.
2. I want to avoid titling a section "Android Studio" since it implies that the first thing to do is install Android Studio. But now in D5624, JDK installation has to happen before Android Studio installation.
3. It makes sense to group the rest of the steps as general debugging tools since that's what they are.

Test Plan: N/A

Reviewers: jon!, atul, varun

Subscribers: tomek, abosh

Differential Revision: https://phab.comm.dev/D5625