Initial implementation

Things that need to be done:

• build docker image with:
  • markdown-cli → https://github.com/igorshubovych/markdownlint-cli
  • markdown-spellcheck → https://www.npmjs.com/package/markdown-spellcheck
  • base configuration file for markdown-cli:

    ---
    MD013:
      line_length: 192
    MD024:
      siblings_only: "true"

• proper documentation
• CI tests and markdown files from this project tests with build container (so building container as first stage, then tests)