doc/Makefile: Allow to create and use .venv #501

Job	Run time
job (debian:stable, s390x, s390x-linux-gnu-gcc, 1, out, cross-compile)	3m 11s
job (debian:stable, gcc, i386)	9m 37s
job (debian:stable, gcc, out, minimal)	9m 13s
job (alpine:latest, gcc)	9m 24s
job (debian:stable, ppc64el, powerpc64le-linux-gnu-gcc, 1, out, cross-compile)	3m 38s
job (opensuse/archive:42.2, gcc)	9m 21s
job (debian:stable, arm64, aarch64-linux-gnu-gcc, 1, out, cross-compile)	3m 49s
job (debian:testing, gcc)	8m 40s
job (ubuntu:bionic, gcc)	9m 48s
job (ubuntu:jammy, gcc, out)	5m 20s
job (fedora:latest, clang, 1)	8m 28s
job (quay.io/centos/centos:stream9, gcc, out)	9m 55s
job (opensuse/leap:latest, gcc)	10m 18s
job (debian:oldstable, clang)	9m 23s
job (debian:oldstable, gcc)	8m 14s
job (debian:testing, clang)	8m 43s
	2h 7m 2s

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