Add a new documentation guide

I think I'd need to be some kind of maintainer to help with documentation, it's not a free access wiki like the old docs system.

@jonathanshaw I added you as a documentation maintainer. You should see an Edit pulldown at the top of the new Guide I just created for the module:
https://www.drupal.org/docs/extending-drupal/contributed-modules/contrib... →

From there you can create pages in the guide, or create "sub-guides" if you think we need them for the submodules. The "Guide" field should be "Entity Registration" for anything you create.

The summary for the Guide I created is now displayed on the main doc listing at https://www.drupal.org/docs/extending-drupal/contributed-modules/contrib... → .

I kept meaning to finish what I started here
https://www.drupal.org/docs/extending-drupal/contributed-modules/contrib... →

And I've been meaning to start my piece, hence the bump. Best laid plans... :)

Created outline: https://www.drupal.org/docs/extending-drupal/contributed-modules/contrib... → . The pages are all empty for now - my thought is many will eventually be a screenshot and explanation.

@jonathanshaw your Permissions page is in the Registration overview section. There is a separate Concepts page now, some of what you have on your page could be moved there. There is also a Configure permissions page in the site builders section, my thinking there is it would be a screenshot and example with some description, with a link to your existing page for those wanting more information.