Doxygen improvements to improve documentation of C API

[ChuckHastings]

Some doxygen improvements:

• Defined a C API module
• Defined submodules for each C API algorithm header
• Moved sampling functions into a separate header
• Moved traversal functions into a separate header
• Deleted a few old nvgraph doxygen things that were interfering with a clean definition of the cugraph namespace

[codecov-commenter]

Codecov Report

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[ajschmidt8]

Suggested change

	sed_runner "s|PROJECT_NUMBER\s*=\s*${CURRENT_SHORT_TAG}|PROJECT_NUMBER=${NEXT_SHORT_TAG}|" cpp/doxygen/Doxyfile
	sed_runner "s|PROJECT_NUMBER\s*=.*|PROJECT_NUMBER=${NEXT_SHORT_TAG}|" cpp/doxygen/Doxyfile

I think it's a good practice to make the first pattern in these sed expression not depend on any particular version (i.e. ${CURRENT_SHORT_TAG}) because this will prevent the expression from properly replacing values when they're out of sync (which seems like the case here since PROJECT_NUMBER is many versions behind at 0.14).

Disclaimer: I didn't test this expression change locally.

[ChuckHastings]

Updated. I also tested the script locally and fixed an issue in my regex.

[ChuckHastings]

rerun tests

[BradReesWork]

@gpucibot merge