Add a roles and permissions page for InfluxDB 3 Cloud

Follow-up from review of #7338.

In [#7338 the Admin UI page](https://github.com/influxdata/docs-v2/pull/7338#discussion_r3476473428) documents roles inline in a table, and @jstirnaman raised it twice:

> We should probably add a page about roles that we can link to, maybe under `admin/` and/or a `reference/` page about ABAC. That can be a separate issue.

> [Related to my earlier comment,](https://github.com/influxdata/docs-v2/pull/7338#discussion_r3476508283) Roles and permissions probably deserves a separate page.

## Scope

- [ ] Add a roles and permissions page (likely `content/influxdb3/cloud/admin/` and/or a `reference/` page covering ABAC)
- [ ] Link to it from [`admin/users/admin-ui.md`](https://github.com/influxdata/docs-v2/blob/master/content/influxdb3/cloud/admin/users/admin-ui.md) instead of describing roles inline
- [ ] Keep role names consistent with the token types on `admin/tokens/` (admin tokens and database tokens)

## Why

Roles and permissions are referenced from the Admin UI page, the users page, and the tokens page. A single page gives those a link target and a place to document ABAC.

Read content/influxdb3/cloud/admin/users/admin-ui.md and the documentation under admin/tokens/ to identify the existing role and token terminology. Add a roles and permissions page under the proposed cloud admin or reference area, link to it from admin-ui.md, and keep role names consistent with admin and database token types. Done means the shared page exists and the inline roles description is replaced by a link.

Written by the indexing model from the issue text.