expand rapids-cmake cpm docs

[robertmaynard]

Description

Provide a more detailed high level overview of the rapids-cmake cpm functions and what you can do with them

Fixes #https://github.com/rapidsai/rapids-cmake/issues/551

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] The documentation is up to date with these changes.

[robertmaynard]

Requires https://github.com/rapidsai/rapids-cmake/pull/614 for CI failures

[robertmaynard]

/merge