first chapter debian installation instruction are wrong

these will not work on debian 10:
$ sudo apt-add-repository -y ppa:ansible/ansible
$ sudo apt-get update
$ sudo apt-get install -y ansible

you might try this:
apt install gpg-agent
apt-get install software-properties-common
apt-add-repository -y ppa:ansible/ansible

put running apt-get update fails with this repo.

I've ended up just running apt-get install ansible and got the 2.7.7 installed.

using these (from ansible site) gives you 2.9 installed:
Installing Ansible on Debian
Debian users may leverage the same source as the Ubuntu PPA.

Add the following line to /etc/apt/sources.list:

deb http://ppa.launchpad.net/ansible/ansible/ubuntu trusty main
Then run these commands:

$ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 93C4A3FD7BB9C367
$ sudo apt update
$ sudo apt install ansible

Locate the first chapter's Debian installation instructions and compare the listed repository commands with the Debian 10 and Ansible installation guidance in this issue. Update the instructions so the commands work on Debian 10 and clearly produce the intended Ansible version, then verify the revised steps.

Written by the indexing model from the issue text.