tools/docs/sphinx-build-wrapper: allow skipping sphinx-build step

tools/docs/sphinx-build-wrapper: allow skipping sphinx-build step

Most targets have two steps:
- step 1: run sphinx-build;
- step 2: run a post-build logic.

The second step can be as simple as copying static files like CSS,
but may may also envolve running make. allowing to skip the first
step helps debugging what's broken, and also allows using make
command line arguments like --ignore-errors.

Add an option to skip step 1.

Requested-by: Akira Yokosawa <akiyks@gmail.com>
Link: https://lore.kernel.org/linux-doc/5031e0c4-f17e-41b8-8955-959989e797f2@gmail.com/
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <30dae295606ae1735f3bbeef4ca041a76dcd4540.1758539031.git.mchehab+huawei@kernel.org>