Add CONTRIBUTING.md for new contributors

[tanikush]

Hi! I cloned this repo to try it out and noticed there's
no CONTRIBUTING.md.

New contributors (like me) have to piece together the dev
workflow from README + scripts manually.

I'd like to add a CONTRIBUTING.md that covers:

• Local setup steps
• How to run tests (pytest in worker/)
• How to validate Terraform (validate-terraform.sh)
• Commit message conventions already used in this repo

Would this be a welcome contribution? Happy to submit a
PR if you'd like.

[tanikush]

While exploring the repo further, I also noticed that none of the
Terraform modules in modules/ have a README.md explaining their
inputs, outputs, and purpose.

I'd like to include that in the same PR:

• CONTRIBUTING.md (as mentioned above)
• modules/alb/README.md
• modules/network/README.md
• modules/observability/README.md
• modules/storage/README.md
• modules/worker/README.md

Happy to keep them as separate PRs if you prefer. Let me know!