Stdlib doc block parsing needs to change

[chenglou]

Now that we use markdown for stdlib docblocks (see rescript-lang/rescript#5095), our parsing of the api docs pages will need to change. Thankfully mostly simpler.

• This only applies to Belt and Js. Pervasives isn't touched.
• Most semantics are still preserved, except
  • {!MyModule.myValue} links
  • Headers. Will have to think about this.

[fhammerschmidt]

@aspeddro do you know if this is still relevant?

[aspeddro]

I don't think it's relevant.