Add documentation for adding new Pages users

[drewbo]

cloud.gov Pages doesn't have documentation on how to manager users and it's leading to some confusion (especially since we still haven't deprecated the collaborators tab). We should add a page to our documentation which clarifies the following:

• Users need to ask their organization manager for access
• Organization managers can add new users via the pages -> organization pane -> edit organization -> add new user
  • Screenshots would be helpful here, especially until we get improved UX
• CMS users still need to login to pages.cloud.gov first before using the content editor

Acceptance Criteria

• New documentation post exists detailing the above information

cc: @cloud-gov/pages-ops

[svenaas]

@Ephraim-G, how can I help with this?

Note that we have now removed the Collaborators tab: cloud-gov/pages-core#4085 (in production, even, as of cloud-gov/pages-core#4086).