[BUG]: Doc issues do not fail CI

[gevtushenko]

Is this a duplicate?

• I confirmed there appear to be no duplicate issues for this bug and that I agree to the Code of Conduct

Type of Bug

Something else

Component

Infrastructure

Describe the bug

#7052 fixed a number of bugs in CCCL docs that got through CI. We should make errors like these a CI failure. @vyasr suggested using WARN_AS_ERROR=FAIL_ON_WARNINGS in doxygen config and -W for sphinx-build.

How to Reproduce

Revert mentioned PR and see build logs

Expected behavior

Bugs in docs do not get through CI

Reproduction link

No response

Operating System

No response

nvidia-smi output

No response

NVCC version

No response