media: doc: properly format ToC headings

"class:: toc-title" was a workaround for older Sphinx versions that are
no longer supported.

The canonical way to add a heading to the ToC is to use :caption:.
Do that.

Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: linux-media@vger.kernel.org
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231027081830.195056-10-vegard.nossum@oracle.com

media: docs: use the new SPDX header for GFDL-1.1 on *.rst files

SPDX v3.10 gained support for GFDL-1.1 with no invariant sections:

https://spdx.org/licenses/GFDL-1.1-no-invariants-or-later.html

So, remove the license text, replacing them by this new SPDX
license.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

media: docs: move uAPI book to userspace-api/media

Since 2017, there is an space reserved for userspace API,
created by changeset 1d596dee3862 ("docs: Create a user-space API guide").

As the media subsystem was one of the first subsystems to use
Sphinx, until this patch, we were keeping things on a separate
place.

Let's just use the new location, as having all uAPI altogether
will likely make things easier for developers.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>