docs: reference book in API ref and README.md, replacing redundant sections

[Trouv]

Now that the "introductory chapters" of the book are complete, the book can now be referenced by other repository documentation. In particular - the getting started guide and explanation of "game logic integration" were both already existing in a lesser form in the API ref and README.md. These sections have been replaced by references to the books documentation, and the tile-based game tutorial/game logic integration chapters respectively.

[Trouv]

Since this introduces temporarily-broken links in the README.md until 0.9 is released, I'm considering holding off on merging this until the release is nigh. A bit annoying in the meantime but at least it won't confuse people.