melange packages should be more prominent

[davesnx]

I propose Js, Belt and Stdlib to be part of the sidebar and some pages to have specifically a nice documentation:

• Specifically for Promise (related https://github.com/melange-re/melange-re.github.io/issues/61)
• Specifically for JSON

Aside from being more prominent, we should give a little opinionated context on each:

• Stdlib (for the people used to OCaml’s stdlib or for the projects that need to be compatible with both tragets?) (compiles to 0)
• Belt is a thin layer on top which gives a more idiomatic approach (non zero binding kind of thing)
• Js library matches perfectly with https://developer.mozilla.org/en-US/docs/Web/JavaScript (with it's quirks)