Use CSS instead of recommending changing browser defaults

[eggrobin]

This pull request adds a CSS stylesheet which appends Assurbanipal to the default fonts, and uses the font Santakku for anything with the class ob-cursive should you want to show some OB cursive glyphs (other styles could of course be added similarly).

It changes the instructions in How to interact with this book accordingly.

Note: I have not been able to rebuild the book without spurious changes, thus I have only included the generated changes to the HTML files; you should probably rebuild yourself in order to refresh the search index etc.