Remove nx-cugraph from this repo

[eriknw]

This is part of the migration to https://github.com/rapidsai/nx-cugraph

This is meant to fully remove nx-cugraph except for docs, which are still built from this repo.

CC @nv-rliu @jameslamb @rlratzel

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[eriknw]

Note that I changed some links in the README and a couple of the docs.

[jameslamb]

Thanks! I searched this branch like this:

git grep -i nx

Found just one other thing that probably could now be removed:

cugraph/ci/test_wheel.sh

Lines 26 to 28 in a5fdc8b

	# FIXME: Adding PY_IGNORE_IMPORTMISMATCH=1 to workaround conftest.py import
	# mismatch error seen by nx-cugraph after using pytest 8 and
	# --import-mode=append.

Do whatever you want with that.

Marking this "Approve" because otherwise everything here looks great to me!

[eriknw]

Thanks! I searched this branch like this:

git grep -i nx

Found just one other thing that probably could now be removed:

cugraph/ci/test_wheel.sh

Lines 26 to 28 in a5fdc8b

	# FIXME: Adding PY_IGNORE_IMPORTMISMATCH=1 to workaround conftest.py import
	# mismatch error seen by nx-cugraph after using pytest 8 and
	# --import-mode=append.

Do whatever you want with that.

Ha, I left a similar comment here: #4748 (review)

I think we can try removing it once we get CI to pass, which I think will need rapidsai/devcontainers#417 or rapidsai/devcontainers#418