[Multipage docs] Adapt export to PNG

[brauliodiez]

The current export behavior for multipage cover a minimum functionallity:

• It just exports the current page.

This can be good enough.

It wouldn't be a bad idea to start thinking about:

• Showing a modal, indicating, export current page, all pages.
• Export it as PNG/SVG or PDF

In case you choose PNG:

• If is just one image generate the PNG file, if not generate a zip file with all the PNGs (there are good zip file libraries available in the front end for this).

We can break down into subcases.