Documentation for PDF.js 2.0

[Rob--W]

The current master branch features many backwards-incompatible changes (notably the removal of the PDFJS global). The documentation at https://mozilla.github.io/pdf.js/examples/ still shows examples using PDFJS though. Furthermore, the "Full source" link at that page links to the master branch, which does show the expected API usage. This is confusing.

Before officially releasing 2.0, the documentation at docs/ should be updated, e.g. https://github.com/mozilla/pdf.js/blob/c33bf800cc87941cc681c3c54279ab1b9745650d/docs/contents/examples/index.md