Replace "Document" with "Page" in the Index Application

There are plenty of references to "document" in DocumentTreeTranslations.xml for example.

Note 1: We should use "Page" instead of "Document"
Note 2: We should use uppercase "P" for Page IMO since a Page in XWiki has a special meaning and it's not a page like in a book for example. I believe we should use "Page" everywhere instead of "page". We had this discussion before AFAIR. evalica would you like to follow up on this?