Hi,
In the docs, 1. Basics, “requirements”, “composer” is not mentioned.
Never-the-less, (on Ubuntu) “composer” is a separate package that you need to install with the Ubuntu package manager, in order to utilize this methode.
You might want to list it as an (optional) requirement, since some newbies will not have any knowledge of the composer.
The GitHub repository does specify installing through Composer, but Composer itself is not a dependency for installing, running or upgrading Grav. This can all be done through archives published as releases or pre-built packages.