Merge pull request #1 from contao/3.1

Update
Chapter 3 : Update "style-sheet.jpg"
Chapter 3 : Update "protected-module.jpg"
Update "access-rights.jpg"
Chapter 4 : Update "protected-element.jpg"
Merge branch 'update-img-access-rights' of https://github.com/lionel-m/docs into 3.1
Merge branch 'update-img-protected-element' of https://github.com/lionel-m/docs into 3.1
Merge branch 'update-img-protected-module' of https://github.com/lionel-m/docs into 3.1
Merge branch 'update-img-style-sheet' of https://github.com/lionel-m/docs into 3.1