Update GETTING_STARTED.md

[johangithub]

I just went through the onboarding and noticed some road blocks, so updating the onboarding doc to reflect the fixes. A couple things I wasn't sure about

1. Do we want modal installed globally with pipx or do uv run modal in the repo?
2. What I got from the instruction was we set two separate R2 tokens, is that correct?
3. What do you think about using ENV variable and using a script to generate .tfvars?

[greptile-apps]

Greptile Overview

Greptile Summary

Updates onboarding documentation with practical improvements based on real user experience. Fixes incorrect repository name references (open-inspect → background-agents), clarifies credential setup steps, and adds helpful tips like bulk secret upload via gh CLI. The changes improve clarity around Modal CLI installation with pipx, Cloudflare R2 setup requirements, and GitHub App configuration.