Convert overview "\page"s that are intended to be available under "groups.html" to "\group"

QDoc allows the grouping of semantically related elements under a "group", through the "\ingroup" command.

A page documenting the meaning of a group can be produced through the "\group" command, further containing a reference to each element associated with it.

One particular group that is used in our documentation is the "groups" group. The "groups" group is a group that references other group pages in our documentation in order to provide an entry-point to many of our overview documentation pages.

Currently, due to the recent changes in QTBUG-104238, each class/qmltype page now references the group pages that it is related to, such as to provide greater discoverability for them.

Currently, the "groups" group does not only contain group pages. Indeed some actual "\page"s are element of this group.

With the recent changes, this now means that there is a path, from the page of the "groups" group, that will bring the user to some class/qmltype pages that provides no link back to the page that the user comes from, contrary to what the user might experience with other pages under "groups", providing for a confusing experience.

For example, Container Classes is such a page.

To avoid this confusion, those "\pages" could be converted to "\group" pages. This should be possible without having to change the page itself, albeit some restructuring may be propedeutic to the change if only to account for the list of item that will be introduced in the page.