Nice catch for the broken link, I’ll open a ticket for those to be fixed 
Do I need to add this if I use
@forestadmin/agentto be able to extend forest routes or it works out of the box?
Our new version of the agent was designed without any route override in mind, to improve the developer experience.
Depending on your use-case, you should be able to use the Relationships/Fields functions, but for very complex workflow your could also use our Hooks.
Feel free to share the use cases for routes override, I’ll be happy to help 