Get PDF support working again

[transitive-bullshit]

There are numerous build issues with react-pdf and Next.js.

See wojtekmaj/react-pdf#136 and wojtekmaj/react-pdf#874 for the two main ones I'm running into.

I've decided to remove react-pdf from react-notion-x for now until these issues are sorted.

[AnshumanFauzdar]

Instead of leaving blank some coming soon graphics can be displayed otherwise it is plain and confusing for beginners :)

[herohoro]

We look forward to PDF support.(´；ω；｀)

[transitive-bullshit]

This should be working again as of v4.19.0.

I also added an example of how to use pdfs: https://github.com/NotionX/react-notion-x/blob/master/examples/full/components/NotionPage.tsx#L31

[herohoro]

Thank you very much. I will update it as soon as possible.