Fossil (https://fossil-scm.org/home/doc/trunk/www/index.wiki) solves this pretty nicely. You can have documentation as files or in a special wiki namespace and it's versioned both ways, and every repository clone gets everything. Even better than that, your in-tree documentation files are rendered and browseable in exactly the same way as the dedicated wiki namespace.

The linked URL to the home page there can even serve as an example: the "trunk" is a check-in name (https://fossil-scm.org/home/doc/trunk/www/checkin_names.wiki) that points to the newest check-in on the "trunk" branch. You can replace it with any other reference to get the old version; eg, version-2.20 would work to get the version 2.20 of the docs, 2015-03-14 would work to get the version from 14 March 2015, etc.