Update c2-setup.md

[Necklaces]

This PR includes:

• Update c2-setup.md to work with latest version of voltlang:
  • update example to use battery.toml instead of battery.txt
  • add folder structure ascii visualization for clarity
• Update c2-setup.md with Arch linux specifics:
  • Seemingly you need clang and gdc to compile
  • Add note about installing battery from AUR