docs: Update Sphinx requirements

Commit f546ff0c ("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>