Maintenance: improve block-manipulation tests, publishing executor, docs, PR template

[FabijanC]

Usage related changes

• None

Development related changes

• Use a stronger Docker executor for the publishing task on CircleCI:
  • Large (4 CPUs) instead of Medium (2 CPUs)
  • Should speed up publishing (currently takes 24 minutes)
• Improve PR template
• Improve docs
• Improve block-manipulation tests after Abort blocks #388:
  • Add BackgroundDevnet::get_latest_block_with_tx_hashes and replace all occurrences of manually doing it.
  • Rely on the block hash returned by BackgroundDevnet::create_block instead of manually fetching it.
  • Rely more on FieldElement instead of serde_json::Value.
  • Rely more on u64 instead of Option<u64>.

Checklist:

• Applied formatting - ./scripts/format.sh
• No linter errors - ./scripts/clippy_check.sh
• No unused dependencies - ./scripts/check_unused_deps.sh
• Performed code self-review
• Rebased to the last commit of the target branch (or merged it into my branch)
• Documented the changes
• Linked the issues which this PR resolves
• Checked the TODO section in README.md if this PR resolves it
• Updated the tests
• All tests are passing - cargo test