spiffe.io/book link is unavailable

[elinesterov]

as @evan2645 mentioned during the last sig-community call initially, we thought that the "Solving the bottom turtle" book will be hosted separately and not on the spiffe.io website. Thus thebottomturtle.io domain was used for it.
Later spife.io/book section was added, and it referenced https://thebottomturtle.io/Solving-the-bottom-turtle-SPIFFE-SPIRE-Book.pdf .
Since the pdf was hosted on 3rd party website that had an expired certificate pdf became unavailable and later I updated thebottomturtle.io to redirect to spiffe.io/book which completely broke book availability.
The best way to solve these issues is to consolidate all assets under spiffe.io. We should put a pdf of the book as a static resource and fix the links.