Is the Git Book part of the git repo?

Is the Linux Doc Projec part of the kernel?

No. For good reasons. The only people who insists all doc must live in the same repo as the code are the ones who does not value documentation.

Note, that in both examples above there is a documentation in the main repo, but not all documentation lives there.

There are two main options, put your docs in the repo, or throw them all over the floor. Many companies opt for the floor.