[FR] Merging multiple pages when exporting to PDF

### Description

_Since the PDF export is planned (https://github.com/AppFlowy-IO/AppFlowy/issues/2938, https://github.com/AppFlowy-IO/AppFlowy/issues/4054), I would like to emphasize that some options are generally expected from the user._

Indeed, being able to merge multiple AppFlowy's pages to one PDF is a must-have.

### Impact

_AppFlowy could become an easy-to-use documentation writer tool if equipped with a highly functional PDF export.
And from what I've read during my searches on the internet, many people are also looking for a solution to this lack on the market._

This feature would make it easier to organize documents rather than using a single page, whose monolithic appearance makes it difficult to modify and maintain it. Moreover, the exported PDF would be much better organized, rather than lining up all the content in a row without any page breaks.

### Additional Context