docs: move Linux logo into a new `images` folder

docs: move Linux logo into a new `images` folder

Having assets in the top-level `Documentation` directory can make
it harder to find the documents one needs, especially if we want
to add more of them later on.

Instead, create a new `images` folder inside it that is used
to hold assets such as logos.

In addition, update the reference in `scripts/spdxcheck-test.sh`.

Link: https://lore.kernel.org/lkml/8735hicoy7.fsf@meer.lwn.net/
Suggested-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/r/20220528153132.8636-1-ojeda@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>