Don't host the book pdf inside this repo

[gz-c]

Remove the pdf of the book from this repo. Its in the assets/ folder.

Instead, link to the pdf copy that is on the github releases page for CX:
https://github.com/skycoin/cx/releases/tag/book-v0.5
https://github.com/skycoin/cx/releases/download/book-v0.5/cx-programming-language.pdf

[amherag]

The source and releases will now be hosted in this repository: https://github.com/skycoin/cx-book. A PDF to the latest book version can be found here: https://github.com/skycoin/cx-book/releases/download/0.5/CX_Programming_Language.pdf