Documentation: sphinx: replace "Introduction" chapter heading with page title

Replace first chapter heading ("Introduction") with page title named
"Using Sphinx for kernel documentation". This way, the first-level TOC
for doc-guide contains title instead of chapter headings for this page.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Tony Nguyen <anthony.l.nguyen@intel.com>
Cc: Vinod Koul <vkoul@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Cc: Akira Yokosawa <akiyks@gmail.com>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-kernel@vger.kernel.org
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/doc-guide/sphinx.rst

 .. _sphinxdoc:
 
-Introduction
-============
+=====================================
+Using Sphinx for kernel documentation
+=====================================
 
 The Linux kernel uses `Sphinx`_ to generate pretty documentation from
 `reStructuredText`_ files under ``Documentation``. To build the documentation in