[C++] Include an example of converting SNAP datasets to GraphAr format

[lixueclaire]

Proposed changes

This PR provides a sample conversion of SNAP datasets into GraphAr format, showcasing the process with the Facebook graph as a case study. The conversion leverages GraphInfo constructors and the high-level writer functions from the C++ library.

Checklist

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• [x] Lint and unit tests pass locally with my changes
• [ ] I have added tests that prove my fix is effective or that my feature works
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Further comments

As part of #275

[acezen]

Shell we upload the generated snap dataset of graphar format and provide a python download script to download it, like HuggingFace did? That would easy for user to get the data directly and no need to convert by themself.

[lixueclaire]

Shell we upload the generated snap dataset of graphar format and provide a python download script to download it, like HuggingFace did? That would easy for user to get the data directly and no need to convert by themself.

Certainly! This Pull Request serves as a demonstration of utilizing the GraphAr API for conversion. In the future, we plan to make generated datasets from sources like SNAP, LDBC, or others readily available to users. These datasets will be accessible for direct download via a dedicated repository or a specialized website, as detailed in discussion #275.

[acezen]

Can you add some document about the example to docs?

[lixueclaire]

Can you add some document about the example to docs?

Sure, I've created documentation for the example. Could you please review it?