Fix libcuproj doc example and make tests consistent.

[harrism]

Description

This fixes libcuproj documentation example inconsistencies raised in #1500.

It removes the (outdated) example from the doxygen main_page and replaces it with a link to the readme.

It also makes the type definitions simpler in the README example and makes the Test code match the example.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[isVoid]

@harrism I think you just need to merge the changes from #1505 to get the CI passing.