inf7 requires ruby (it was developed on ruby 3.0.1). If you want to be able to generate an epub of the documentation, you'll need pandoc as well. And if you want pygment-formatted source code for your story and extensions in the Project Index, you'll need Python and pygments.
This is probably the easiest way to install without requiring elevated permissions. Assuming you have $HOME/bin in your PATH:
From source:
$ git clone https://github.com/zedlopez/inf7.git
$ cd inf7
$ export GEM_HOME="$HOME/gems"
$ gem build inf7.gemspec
$ gem install inf7-0.2.2.gem
$ cp inf7.sh "$HOME/bin/inf7"
If you use a different value for GEM_HOME, you'll have to modify $HOME/bin/inf7 accordingly. (I don't recommend using sudo gem install and affecting your system-wide gem ecosystem.)
One way to ensure you have the prerequisites for pygment formatting would be:
$ cd $HOME
$ virtualenv --python=python3 pygments
$ cd pygments
$ source bin/activate
$ pip install pygments
$ cp $GEM_HOME/gems/inf7-0.2.2/i7tohtml bin
And then set i7tohtml to $HOME/pygments/bin/i7tohtml (but use the absolute pathname, not something with $HOME).