Clarify the doc index
Clarify the doc index

With regards to the unstable book, the reference, and the
processes involved.

Also, fix up a link by pointing to the new tracking issue rather than
the older one.

Fixes #41285

aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust