Fix MDX pipeline saveImages step (Qiskit#3128)

This PR adds the docs prefix to the `destFolder` used to store the
images in the MDX pipeline. This was necessary to be able to generate
the latest runtime version in
Qiskit#3127

It also removes the `.gitkeep` file for the learning/courses because
it's not necessary anymore.

Author: arnaucasau

learning/courses/.gitkeep

@@ -52,7 +52,7 @@ export async function saveImages(
   publicBaseFolder: string,
   pkg: Pkg,
 ) {
-  const destFolder = pkg.outputDir(`${publicBaseFolder}/images`);
+  const destFolder = pkg.outputDir(`${publicBaseFolder}/docs/images`);
   if (!(await pathExists(destFolder))) {
     await mkdirp(destFolder);
   }