I know it is not the primary use of Org mode, but does anyone uses org mode for documentation (instead of .md or .rst markdown files)?
If you use Emacs, you will have great editor with powerful preview features (images, formatting, table processor with formulae, code execution inside the text), and the pipeline for generating HTML or PDF will be approximately the same as with .md/.rst (org-mode is even supported by GitLab and GitHub web interface, although not perfectly). But the support of the org-mode outside Emacs is worse than of Markdown and reStructuredText.
Can you explain? I mean, I imagine there aren't really various flavors of Org Mode, but that doesn't make it standardized.
> I imagine there aren't really various flavors of Org Mode, but that doesn't make it standardized.
All the implementations that call themselves org-mode follow the conventions set by the canonical implementation - the Emacs org-mode. While this may not sound like a good reason to call it standardized, the practical implication is a vast difference from what you get with various markdown flavors. In the latter case, the only way to make sure that your markdown is correct is to test it with the target implementation.
The second reason is that there is an actual effort to standardize org-mode - called Orgdown [1]. Org-mode is already more or less uniform across implementations. This effort tries to write it down as a reference. Markdown has a similar effort called CommonMark. But if you want to know why it's different, you have to look at the history of why it isn't called 'Standard Markdown'.