Update "Getting ready for development" to describe patching instead of github PRs

[lisastreeter]

A PR has been created for the Commerce 2 / Developer Guide / Contributing page to remove the part that tells people to send github PRs, patches are fine now.

We need to do the same for the "Getting ready for development" page that follows. Also, recommend ddev for all users, to simplify?

Also, update "Running tests" section. Reference Matt's blog posts on the subject: https://glamanate.com/blog/running-drupals-phpunit-test-suites-ddev?utm_source=Drupal%20Planet&utm_medium=feed&utm_campaign=drupal https://glamanate.com/blog/running-drupals-functionaljavascript-tests-ddev