docs: Update Sphinx requirements

Commit f546ff0c0c07 ("Move our minimum Sphinx version to 1.7") raised
the minimum version to 1.7.

For pdfdocs, sphinx_pre_install says:

note: If you want pdf, you need at least Sphinx 2.4.4.

, and current requirements.txt installs Sphinx 2.4.4.

Update Sphinx versions mentioned in docs and remove a note on earlier
Sphinx versions.

Update zh_CN and it_IT translations as well.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Federico Vaga <federico.vaga@vaga.pv.it>
Cc: Alex Shi <alexs@kernel.org>
Reviewed-by: Alex Shi <alexs@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

docs/zh_CN: Add translation zh_CN/doc-guide/sphinx.rst

Add new translation
Documentation/translations/zh_CN/doc-guide/sphinx.rst

Signed-off-by: Wu XiangCheng <bobwxc@email.cn>
Reviewed-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Link: https://lore.kernel.org/r/7c8c2eeb6c0b73410fbdb66cf702dc0e58b02a3e.1618295149.git.bobwxc@email.cn
Signed-off-by: Jonathan Corbet <corbet@lwn.net>