Add `nx-cugraph` Intro to API Docs Page

[nv-rliu]

Closes https://github.com/rapidsai/graph_dl/issues/606

This PR adds a section to RAPIDS Docs > API Docs for nx-cugraph

Things to consider:

Since nx-cugraph is a GPU accelerated backend for Networkx...

• Should we also have separate Stable, Nightly, Legacy pages for rapids docs?
• Is the entire section in the right place / is the wording for the description apt?

We need an elegant landing page like dask-cudf or cuSpacial for users to easily follow.

Proposed change looks like:

as of 5458bbd

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