Standard command for automated doc generation
It would be useful to have an automated one-command method to generate documentation to be used by library users, tool users and package maintainers.
It should provide:
- generate docs and index file for a whole project
- safe and reproducible: no network access, no execution of runnableExamples or other code
- no need to handpick input files
- no need to handpick output files, images, example .nim files etc i.e. all output goes into a dedicated self-contained directory

This would help implementing #227 and #90