* use git+https:// in the example url because not everyone will have ssh
keys registered on github
* Remove the link to the github tarballs as they do not work. users can
use releases.ansible.com instead.
* #27045 fix installation guide via apt (ubuntu)
* Update intro_installation.rst
Re-added apt-get update as it is needed to get the latest package index from ppa:ansible/ansible
Move to using a requirements.txt to install the python packages.
This makes it easy to keep the documentation and actual package
dependencies in sync.
Fixes#18453
