Add integration modules

Ok adding in the key D8/CiviCRM integration modules. I'm thinking just their official drupal.org project page and official docs is more than enough.

Fine in principle - but maybe not in install section? - want to keep to clean installation instructions without tangents for that section :-) . There will be a separate "setup" section in this guide where they should go and probably a reference/link in the Planning section?

You said ok to this earlier with a note to keep it short - which I did :-)

Yeah I did, been rethinking a little on the structure. Let's add it for now - we can always move it if needed.

merged

mentioned in commit 6322e0d2