Revamp OSS user documentation

We are getting a lot of user feedback about the user management docs for InfluxDB OSS. These docs haven't been changed much since the original 2.0 alpha release (before the user system was really completely understood and/or finalized).

We need to clarify the relationships between users, organizations, members, and tokens.

- Users exist outside of the scope of organizations
- Users are added to an organization and become a member of that organization
- Tokens grant permissions to users but are scoped to organizations (other than operator tokens)
- Users can exist in the system, but may not be a member of any organization.
- etc.

We should take a long look to clearly explain how these all work together. At one point, we had even talked about doing away with "user" management docs and leave it at users. With the existing InfluxDB toolset, I don't think that'd be a good move yet.

##### Relevant URLs
- https://docs.influxdata.com/influxdb/latest/organizations/
- https://docs.influxdata.com/influxdb/latest/users/
- https://docs.influxdata.com/influxdb/latest/organizations/members/

Start by reading the linked Organizations, Users, and Organization Members documentation pages and compare how they describe users, organizations, members, and tokens. Map the relationships and identify outdated or conflicting explanations. Done means the OSS user-management documentation clearly explains these concepts, including users without organization membership and the scope of tokens.

Written by the indexing model from the issue text.