Make repo more informative to new users and contributors

[leha-bot]

We should make this repo after these tasks will be resolved:

• License (Boost/Blue Oak, Add license and SPDX identifiers  #7 , Add license and SPDX identifiers #13 );
• Contribution Guidelines;
• Proper docs (via Doxygen/Sphinx/Breathe);
• Polishing code;
• CI/CD (Add GitHub Actions / Another CI/CD #2 , feat(CI): Test Github Actions on Windows and Macos (Take #2) #14);
  - [ ] Version > 0.1.x;
• Another.

[leha-bot]

Unfortunately we were forced to open repo prematurely due to severe CI issues (see #12 for gory details)