First version of consolidated documentation

[joaquinvanschoren]

This PR creates a documentation website that consolidates the different pieces of documentation into a single website, based on MkDocs.

Next steps:

• Make any changes to the website structure in the .yml file.
• After merging, set up a GitHub action that automatically rebuilds the website with every repo update, and hosts it on GitHub Pages
• Ask mlcommons to reconfigure GitHub so that https://mlcommons.github.io/croissant points to the croissant GitHub Pages
• Add a clear link/button from https://mlcommons.org/working-groups/data/croissant/ to this documentation

After that, incremental updates can be made, for instance:

• Adding additional extensions (bio, geo)
• Adding example datasets described in Croissant
• Adding a page with example use cases
• Adding an API reference for the mlcroissant library

[github-actions]

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