doc: introduce sphinx extlinks #609

Job	Run time
job (debian:stable, gcc, i386)	6m 34s
job (debian:stable, ppc64el, powerpc64le-linux-gnu-gcc, 1, out, cross-compile)	5m 5s
job (debian:stable, arm64, aarch64-linux-gnu-gcc, 1, out, cross-compile)	5m 8s
job (debian:stable, s390x, s390x-linux-gnu-gcc, 1, out, cross-compile)	4m 53s
job (alpine:latest, gcc, asciidoctor)	4m 39s
job (debian:stable, gcc, out, minimal)	4m 46s
job (opensuse/archive:42.2, gcc)	6m 4s
job (quay.io/centos/centos:stream9, gcc, asciidoc-pdf, out)	4m 9s
job (debian:testing, gcc, asciidoctor-pdf)	7m 5s
job (debian:oldstable, clang)	7m 15s
job (fedora:latest, clang, 1, asciidoctor-pdf)	6m 46s
job (opensuse/leap:latest, gcc, asciidoc-pdf)	5m 56s
job (debian:oldstable, gcc, asciidoctor)	5m 50s
job (debian:testing, clang, asciidoctor-pdf)	8m 25s
job (ubuntu:jammy, gcc, asciidoctor, out)	4m 5s
job (ubuntu:bionic, gcc, asciidoc-pdf)	5m 36s
	1h 32m 16s

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