Instructions for setting up lando for Pantheon sites need work

[joeparsons]

We have some documentation in our README file about how to use lando for running migrations from a Drupal 7 or UA Quickstart site downloaded from Pantheon to a Drupal 9 Arizona Quickstart site downloaded from Pantheon.

The included instructions about how to setup lando locally initially are not very clear. We really need some explanatory text about this line that explains that lando init --source pantheon command needs to be run inside an empty directory instead of inside a clone of the site's git repo:
https://github.com/az-digital/az-quickstart-pantheon/blob/master/README.md?plain=1#L200

It would also be good to add some more generic info about setting up lando outside of the migration specific context.

[trackleft]

See #144 as well