Ready for stable release (in theory)

[MangelMaxime]

The main goal of this PR was to make it ready for stable release for that I:

• Lower FSharp.Core requirements -> related to the discussion in Update Fable.Core and FSharp.Core #126

• Clean up dependencies -> You should only need to depends on Feliz not Feliz.CompilerPlugins.

  Feliz.CompilerPlugins should be controlled by Feliz

I also updated the docs project to use Webpack 5, because this is requirred in order to use Fable.Elmish.HMR latest version. And the project is also updated to Elmish 4.

My only problem is that I can't make the icons work using only the SCSS import. To hack around that I included a CDN version of it via the index.html file. I don't know if this is related to a change I made or if it was broken before.

[Dzoukr]

Hi, it looks good. What do you think about versioning? Should we bump it to a major 3x version so just a minor update?

[MangelMaxime]

This should be a major bump because it relies on Feliz 2, which relies Fable.React 9 (breaking change) and on Fable 4.