Proposal: Restructure DataFusion site #1821

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I am proposing to restructure the DataFusion site drawing some inspiration from the Arrow site (https://arrow.apache.org/docs). Assuming we reach consensus on some variant of this I think this issue would be best served as a parent / tracker since completing this will have many changes.

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I think it would be nice if the structure was something like the below. This is very rough right now (and im not familiar enough with the complete installation to fill in all details) but I think illustrates an overall structure that would be nice - IMHO.

• About
  • Quarterly Roadmap
  • Long Term Roadmap
• Supported Environments
  • Rust
    • Installation
    • Key Interfaces
      • ExecutionContext
      • DataFrame
      • ObjectStore
      • ListingTable
      • ...
    • Internals # I figure since Rust is the foundation for everything else its worth explaining some of the internals???
      • Logical Plan
      • Physical Plan
      • Expressions
      • ...
    • Extensions
    • API Reference
    • ...
  • Python
    • Installation
    • API Reference
    • ...
  • Java???
  • CLI
    • Installation
    • ...
  • Ballista
    • Installation
    • ...
• Cookbook
  • Rust
  • Python
  • CLI
  • Ballista
• Development
  • Rust
  • Python
  • CLI
  • Ballista
• Specification
  • DataFusion's invariants
  • DataFusions output field name semantics
  • SQL Reference
• Community
  • Communication
  • Issue tracker
  • Code of conduct
  • DataFusion-contrib
  • FAQ

I personally really like having Cookbooks to learn the idiomatic / performant ways to perform various tasks (see https://arrow.apache.org/cookbook/py/ as an example). That being said I recognize it would be extra work and likely isnt needed in the short run until the core structure is in place. But in the medium to longer term I think there is real value in it - in particular for new users.

Similar can be said for the Internals section in rust. That being said, I believe @alamb and @andygrove have already produced some presentations / talks that could potentially be leveraged to help fill in those details (on top of whatever is in docs.rs).

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Additional context
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I have a few datafusion features I plan on working on in the short-medium term but after those are completed i would be happy to drive the implementation of this once there is agreement.