contrib/subtree: fix building docs

contrib/subtree: fix building docs

In a38edab7c8 (Makefile: generate doc versions via GIT-VERSION-GEN,
2024-12-06), we have refactored how we build our documentation by
injecting the Git version into the Asciidoc and AsciiDoctor config
files instead of doing so via arguments. As such, the original config
files were removed, where the expectation is that they get generated via
`GIT-VERSION-GEN` now.

Whie the git-subtree(1) command part of "contrib/" also builds docs
using these same config files, its Makefile wasn't adjusted accordingly
and thus building the docs is broken.

Fix this by using `GIT-VERSION-GEN` to generate those files.

Reported-by: Renato Botelho <garga@FreeBSD.org>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>