Going to write a book-brochure to yourself and thus to systematize their knowledge.
Questions in order (you can answer as "true\false"):
1) Correct, if the book will be read&to be supplemented in the reader, the computer & the web (smartphone, etc) - Markdown ideal?
2) Correct that online engines, storage (free) for such booklets not?
3) Right that free desktop applicationthat allows you to text to format using the GUI and save the result in markdown format there, and because of this I will have to learn the markup language (and wrote the text files in my code editor)? (You know, to teach a couple of hours there, but still)
Or share your experience - like store your notes, as sistematizarea knowledge?
No, markdown is not to be considered the best markup language, although its task it is, in principle, cope.
You can store your notes on GitHub.
So if you put the question, select OpenOffice write and use, but it won't be markdown. Or, if you need to store these notes in the network, can use Google docs, also without the markdown.
I should add that the whole point of markdown is precisely in the intended simplicity of the input layout (to remember where you need five minutes) manually. Because it is unclear why you both markdown and GUI for formatting them.
Looks like you went on common way back fit the chosen technology under the ill-defined task, instead of searching for technologies to solve clearly defined.