How to build class documentation?

Hi,

I'm trying to build class documentation for Redmine 4.1.1 but running rake doc as per https://www.redmine.org/projects/redmine/wiki/Contribute#Code-documentation produces nothing in the doc folder.

I also tried to execute rake yard but I receive:

redmine@dev2:~/redmine$ rake yard
rake aborted!
Don't know how to build task 'yard' (see --tasks)

(See full trace by running task with --trace)

I also tried to run export RAILS_ENV=development before running rake yard but I got the same.

Could you help me to understand where I'm wrong, please?

Thank you very much!

Redmine