Expose Contexts using hooks

[iamandrewluca]

Reason: Want to create a Thumbnail component, and need access to the DocumentContext

[wojtekmaj]

I think it's generally a good idea, will review more carefully soon.

In the meantime, just curious: how will thumbnail be different from just Page with additional layers disabled? 🤔

[iamandrewluca]

I looked through pdf.js/viewer and saw that they generate low-resolution images from the page.
I think this may improve performance a little, because when using Page I suppose any change in Document that Page depends upon, will make the page re-execute some logic.
With images there should be no such problem, they are generated once.

If I will find an elegant way to create a Thumbnail component, I will propose to be included in react-pdf

[wojtekmaj]

Oh, now I see it. I think the main problem would be that the Page component "registers" itself in Document component to basically say "hey I'm here" so that whenever you click an internal link in PDF it could scroll to this particular page. Thumbnail should be stripped off from this logic.

[wojtekmaj]

Hey, there were merge conflicts so I went ahead and committed the changes based on your work and added you as a co-author. Hope that's alright :)