[cleanup] Updated document to the new xar format.

(cherry picked from commit d93009c698b5461a8259cc1f46d39a6230f4f63f)
XWIKI-11249: Wrong data space computed for apps when going back and forward in the app creation wizard
- Using an object to store the data space name
- Preserving backwards compatibility with previously created apps

(cherry picked from commit 2e0235da175699c9b6cbe819635fb48eb50626d1)