Generate doc book for latest release as well as latest master

[charypar]

With the current setup, the book at https://redbadger.github.io/crux/ follows master and is always ahead of the lastest released version, which might cause confusion when bigger changes are happening.

We should build two versions of the book, default to the latest release and link to the master version instead. This should be perfectly possible by tweaking the github pipelines.