Improve testing and contributing

[FabijanC]

Usage related changes

• None

Development related changes

• Remove redundant artifact - replaced with utility function
• Minimize a sierra artifact
• Improve contribution guidelines (should have been done before OD Hack, but better late than never)
• Improve Background{Devnet,Anvil}:
  • Use shorter for loop instead of while
  • Reuse client used for connection checking

Checklist:

• Checked the relevant parts of development docs
• Applied formatting - ./scripts/format.sh
• No linter errors - ./scripts/clippy_check.sh
• No unused dependencies - ./scripts/check_unused_deps.sh
• No spelling errors - ./scripts/check_spelling.sh
• Performed code self-review
• Rebased to the latest commit of the target branch (or merged it into my branch)
• Updated the docs if needed, including the TODO section
• Linked the issues which this PR resolves
• Updated the tests if needed; all passing (execution info)