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/[e-mail]/
Suggested-by: Jonathan Corbet <[e-mail]>
Signed-off-by: Miguel Ojeda <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

scripts/spdxcheck-test.sh

···
 #!/bin/sh
 
 # run check on a text and a binary file
-for FILE in Makefile Documentation/logo.gif; do
+for FILE in Makefile Documentation/images/logo.gif; do
 	python3 scripts/spdxcheck.py $FILE
 	python3 scripts/spdxcheck.py - < $FILE
 done