tools/docs: sphinx-build-wrapper: generate rust docs only once

tools/docs: sphinx-build-wrapper: generate rust docs only once

Currently the rust docs are generated for each entry in SPHINXDIRS.
This is unnecessary as they will be the same for each one.

Move the generation, so it is executed only once.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Tested-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260118-docs-spurious-rust-v1-1-998e14b9ed9e@weissschuh.net>