How to link to login page? I am trying to do something a priori simple. 

I wish in a template to add a link to the login page. 

I found this documentation, so I know how to link to a node etc. but to a url type "/ login", I did not find. 

Do you know how to do it ?
You can use the link function and the route for the login page (user.login), e.g.

{{ link('Login', 'route:user.login') }}