Nice to have: enhance user-created repo

Nice to have on the user repo, created with AR.js Studio:

- set Github Pages as 'default' branch
- on the Readme, add the URL of github pages
- output Github repository URL (not only github pages) and add it on the response of the `package.serve` when packageType is `github`
- show marker image on the Readme (using relative path in reference of the repo).

