[FR] Subpage show into parent page

[mobsenpai]

1~3 main use cases of the proposed feature
I make a subpage using the "+" button from the sidebar to a parent page, but if I want that subpage to show up in the parent page as if I make a "document" from the parent page. I want to be able to do that.

what types of users can benefit from using your proposed feature
Who manage a master database, and want to show those subpages in a single page. Making subpage in the page is added already as "+document" but I've already made many subpages before that feature came out, so I would be able to convert that now.

Additional context
I can think of some ways to be able to do that:

1. Making grids able to move from one page to another using drag and drop
2. Making a subpage move to another page using drag and drop

[mobsenpai]

@annieappflowy