For me it is a convenient and compact tool in which I can put all my ideas, goals, thoughts
Evernote - Bring your life's work together in one digital workspace. Evernote is the place to collect inspirational ideas, write meaningful words, and move your important projects forward.
GitBook - Modern Publishing, Simply taking your books from ideas to finished, polished books.
Google Keep - Capture notes, share them with others, and access them from your computer, phone or tablet. Free with a Google account.
DocFX - A documentation generation tool for API reference and Markdown files!
Joplin - Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, tagged and modified either from the applications directly or from your own text editor.
MkDocs - Project documentation with Markdown.