WIP: Save progress made to update the documentation for Fable 4

[MangelMaxime]

Not ready for review yet, I am using the PR to keep track of my work

• Create a section per languages

  I decided to isolate each language section and duplicate the compatibility matrix. It can require a bit more of initial work for us maintainer but it has the benefit of being focused on only the target the user is interesting in.

• [x] Introduce the multiple target support introduced by Fable 4

• [x] Convert the template usage to a step by step explanation on how to setup a Fable project

• [x] Rework the global architecture of the documentation to not be JavaScript focused only

• [x] Have a clear explanation on how bindings works. People, seems to be confused why they need NPM and NuGet packages.

• [x] Remove JavaScript focus from the .NET user introduction and try to explain how Fable integrates in target language toolchain

  • JavaScript
    • [x] Quickstart
    • [x] Build and run
    • [x] Features
      • ~[ ] Named arguments~ Same as ParamObject and I am trying to check if we can remove it from the API
      • [x] ParamObject usage for POJO
    • [x] Compatibility
  • TypeScript
    • [x] Quickstart
    • [x] Build and run
    • [x] Features
      • [x] Erased Union
      • [x] Tagged Union
      • [x] F# Union
    • [x] Compatibility
  • Python
    • [x] Quickstart
    • [x] Build and run
    • [x] Features
    • [ ] Compatibility
  • Rust
    • [x] Quickstart
    • ~[ ] Build and run~
    • ~[ ] Features~
    • ~[ ] Compatibility~
  • ~Dart~
    • ~[ ] Quickstart~
    • ~[ ] Build and run~
    • ~[ ] Features~
    • ~[ ] Compatibility~

[MangelMaxime]

This PR changes the website structure to support multi-languages, the references pages are the one from JavaScript.

I created the section for TypeScript and Python too.

Still left to do is Rust and Dart, but I don't know much about them so I can't really help right now on documenting their features. Also Rust is still in alpha and I believe the interop/binding story is not decided yet.

For Python, while writing this section I found several strange cases that I reported in an issue on Fable repository.

Please feel, free to contribute to your respective section if you think more can be done or if I made mistakes somewhere.

cc @alfonsogarciacaro @dbrattli @ncave