Add markdown documentation-as-code mechanics

[MarcinCelej]

Sample:

           yield return new Markdown.Header3("First step: business requirements gathering");
            yield return new Markdown.Paragraph("Quite often you might hear during analysis phase of your project such words:");
            yield return new Markdown.Quote("When withdraw limit is set, withdrawn amount cannot exceed the limit");
            yield return new Markdown.Paragraph("When you hear such sentence just start your development by writing it down like that:");
```