Remove participants as a search parameter when listing Project Groups
Edit Project Groups Documentation

* Fix casing of resource names
* Rename `projects` parameter to `project_ids`
* Reference the correct objects in sample responses
* Wrap sample responses in JSON arrays when appropriate
* Change behavior of create action: If a project is assigned to an
  existing project group, it will be reassigned to the new project
  group on creation.
  * Include note for `project_ids` parameter
* Change URLs for adding and removing projects from a project group
* Fix URLs of Sample resources to point to `project_groups` instead of
  `groups`
* Remove hypermedia URL to list participants in a project group
* Add hypermedia URL to list projects in a project group