Add MIT License and update documentation and workflows for publication

[hyperfinitism]

To prepare the project to be public, this PR made the following updates:

1. Add MIT LICENSE file
2. Add MIT license SPDX headers to all source files
3. Add SPDX License header check workflow
4. Add CONTRIBUTING.md and SECURITY.md
5. Update README.md — SemVer, Minimum Supported Python Version and LICENSE badges, lint error fixes, change comments in code blocks