Update chapter 08 (Extension repository)

Add new categories.
Add GitHub import in the Extension Repository.
Add new images (github-import.jpg + github-import-tag.jpg).
Update images (add-extension.jpg + edit-files.jpg).
HTTP -> HTTPS
Add new paragraph
List of modules (improvements)

The list has been completed.
Fixes some CSS class names (confusion with the template names).

Reference : https://contao.org/en/manual/3.0/managing-pages.html#modules
List of modules (modifications in German)

The list has been completed.
Fixes some CSS class names (confusion with the template names).

Reference : https://contao.org/de/manual/3.0/managing-pages.html#module
Chapter 8 (update in German)

Add new categories.
Add GitHub import in the Extension Repository.
Add new images (github-importieren.jpg + github-importieren-tag.jpg).
Update images (dateien-bearbeiten.jpg + erweiterung-hinzufuegen.jpg).
Merge branch '3.0' of git://github.com/lionel-m/docs into 3.0