replace enable: false with draft: true

[hexylena]

Leaving a tutorial disabled maybe feels more judgemental (and our phrasing in the notification box), we could rephrase this as draft mode which might make it more comfortable for authors. Suggestion by @shiltemann

[hexylena]

edit: draft mode is literally part of jekyll already, I do not know why we didn't use that. There's a flag --drafts to include drafts in the cli, published in the frontmatter controls it.