Update SDDS docs to have section for xb-demo install

add parent

This is unblocked now.

Updated the dev guide and testing guide:

https://www.drupal.org/community-initiatives/starshot-demo-design-system... →
https://www.drupal.org/community-initiatives/starshot-demo-design-system... →

Marking fixed but feel free to still review

Closing... you can still add a comment though if something isn't correct :) [or ping me in slack]