Document unified operators

[cometkim]

As soon as rescript-lang/rescript#7057 landed

We need to fix the explanation for operators. It seems better to fix the existing one instead of introducing it as a special feature. (because nothing looks special 😂 )

Also be good to add a small note about the type inference rules, like F# does.

[cometkim]

Now we have the blog post https://rescript-lang.org/blog/introducing-unified-operators

I'll work on docs update too