book: dead link to timber & TODO

[Ralph Ursprung]

on the start page of the book there's a dead link to Timber (pointing to https://timber-lang.org/ which is now just a domain parking).
also, there's a TODO in the same chapter:

For a full list of related research see [TODO].

and in the RTIC into the Future there's another TODO at the end:

For more details and examples see Section [todo].