.gitignore: docs: ignore sphinx_*/ directories

The default way of building documentation is to use
Sphinx toolchain installed via pip, inside the
Kernel tree main directory. That's what's recommended by:

	scripts/sphinx-pre-install

As it usually provides a better version of this package
than the one installed, specially on LTS distros.

So, add the directories created by running the commands
suggested by the script.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/ac4e23d556c7d95cb11d6d5c605f43e425b2c3c7.1599660067.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

.gitignore

@@ -152,3 +152,6 @@ x509.genkey
 
 # Clang's compilation database file
 /compile_commands.json
+
+# Documentation toolchain
+sphinx_*/