How to obtain the complete set of man pages from man7.org on a Linux machine?

I’m not aware of a single source for all the man pages on man7.org, but there is a list of all the source projects used to build the web site; if you install all the corresponding packages, you’ll have all the relevant man pages.

In particular, you should install the manpages and manpages-dev packages; these contain the man pages maintained as part of the man-pages project.

Note that the web site reflects the current state (or latest release) of each source project; this will be newer in many cases than the versions available in Debian 9 or 10. In particular, you won’t find the veth man page in Debian 9: it was added in version 4.14 of the man-pages project, but Debian 9 has version 4.10. Version 4.16 is available in Stretch backports.

Install the manpages from stretch-backports:

printf "%s\n" "deb http://deb.debian.org/debian stretch-backports main" \
|sudo tee /etc/apt/sources.list.d/stretch-backports.list
sudo apt update
sudo apt install -t stretch-backports manpages
man veth

veth manpages : stretch-backports