Extract map starter kit and refactor CI

This extracts the workadventure map starter kit (used the most recent version of the workadventure template from GitHub) into its own subdirectory for reference, but keeps only the files we need for our rc3 workadventure in the root.

Also added a README more specific to us. :)

The CI now builds the test-page for testing purpose, but for deploying the optimized maps, this is irrelevant. So this CI job is more to avoid people breaking the local test possibility for others.