rapids-export correctly reference build code block and doc strings

[robertmaynard]

The update to rapids-cmake, introduced two regressions that this PR fixes.

1. re-include the a documentation header to the cudf-config.cmake files describing the project
2. The build directory cudf-config.cmake properly recreates testing dependencies and the cudf::Thrust target

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (branch-21.10@8a3efd0). Click here to learn what that means.
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[harrism]

@gpucibot merge