Doc: Update installation guide to avoid 'pipx'

[ClausHolbechArista]

Change Summary

Update installation guide to avoid 'pipx'

Repository Checklist

• My code has been rebased from devel before I start
• I have read the CONTRIBUTING document.
• My change requires a change to the documentation and documentation have been updated accordingly.
• I have updated molecule CI testing accordingly. (check the box if not applicable)

[gmuloc]

LGTM

For posterity sake:

I read pipx doc and basically you would need to:
1/ install ansible with pipx (it creates a venv for ansible AND make ansible command globally available)
2/ inject requirements.txt inside you ansible using pipx inject
3/ point your python interpreter variables in ansible playbook to the venv python interpreter
4/ it should work

I have not tested and feel like people should rather use a venv rather than pipx :)

[carlbuchmann]

LGTM