https://en.wikipedia.org/wiki/Markdown

There is no official Markdown specification and there are many different
implementations.
This seems intended by the creators of the language.

TODO

• count number of headings for each heading level (loop through all headings, find the highest heading level, get all used levels
  then create a slice or slice of maps and append after each deeper level discovered...)
• print heading tree
• delph deeper into different Markdown implementations (like CommonMark)
• add support for "Setext-style headers"