docs: Update contributing guidelines and development workflow

[bcdady]

Changes

• chore: add GitHub issue templates and config

  • In anticipation of greater community collaboration, we want to provide templates to help clarify expectations to other contributors for how they can explain bugs they may encounter, features they may wish to promote, or other opportunities for improvement of tfsumpy

• docs: update contributing guidelines, move tests guidance to a new doc, and link to tests doc from contributing guidelines

  • Move Code Style, Testing, and Documentation sections up in the contributing guidelines. This is intended to provide a new contributor with context before they get into specific instructions.
  • Break Testing concepts into 3 sections: 1 for introducing the preferred testing strategy for this project -- creating tests before starting feature development (TDD). The 2nd describes expectations on running tests locally, in order to ensure a successful test pass after the branch is pushed and a PR is opened. Additionally, the new tests.md provides a quick intro to anyone new to pytest and/or adding/updating tests for this project.
  • Add reference links to ensure new contributors can understand code and documentation styles
  • Explain Taskfile and reference how to use commands in Taskfile when possible, to maintain consistency across contributors as well as with CI workflows

Testing

• test: bump up coverage threshold to 80, to match/restore the original Testing target
  • "Maintain test coverage above 80%"

Related Issues

• Closes Provide GitHub issue templates #23