Fetch all manpages available in packages into the current directory

fetch-manpages.sh

#!/usr/bin/env -S bash -euo pipefail

#
# Fetch all available manpages.
#

n=$(apt-cache pkgnames | wc -l)
i=0
for e in $(apt-cache pkgnames)
do
    echo "Fetching $e ($((++i)) of $n)" >&2
    apt-get --print-uris download $e \
        | perl -ne 'print "$1\n" if /^'"'"'(.*?)'"'"'/' \
        | xargs -r curl -sL -o- \
        | dpkg-deb --fsys-tarfile /dev/stdin \
        || :
done \
    | tar xv --ignore-zeros ./usr/share/man \
    | perl -pe 's/^/+ /'

Quickstart:

$ mkdir /usr/local/share/extra-manpages
$ wget https://gist.githubusercontent.com/NuvandaPV/fc15eb998c305f8fc8123bf471971e79/raw/be4a4a93665875b3699ba1eb24a5f778950cb8de/fetch-manpages.sh
$ chmod +x fetch-manpages.sh
$ ./fetch-manpages.sh

As /usr/local/share/extra-manpages/usr/share/man is usually configured to automatically be included in the manpath, this should be all that is needed.