Improve docs for users and roles

[cjcenizal]

RBAC docs

The RBAC docs contains out-dated screenshots. This page should cross-link to the Manager user docs (below) to guide the reader on how to define administrator-level permissions.

This page should also explain what the "Default role" is for.

In terms of naming, this page might also be more clearly titled "Corpora access permissions", because that's what RBAC is used for. We should still describe how RBAC is used in the content itself, but we can make the title more accessible by framing it in terms of the workflow that's relevant to the user, not the implementation/technical details.

Manager User docs

The Manage User docs also contains outdated screenshots, but it also doesn't explain what the roles do (Account Admin, Corpus Admin, or Billing Admin). This page should cross-link to the RBAC docs to guide the reader on how to control user access to specific corpora.

I recommend creating a "Administration permissions" page as a sibling to the above RBAC docs page, which will explain what these roles do, and then cross-link to the "Administration permissions" page from the Manager User docs page.