Allow introspecting contracts in the generated documentation

[thufschmitt]

(the title of the issue is rather bad, apologies about that)

Is your feature request related to a problem? Please describe.

Given a Nickel expression like

let TypeOfFoo = { _: String } in
{
  foo | TypeOfFoo
}

the doc output will look like this:

foo

• foo | TypeOfFoo

This is only marginally useful since I have no way of knowing what TypeOfFoo is without looking at the code.

Describe the solution you'd like

Anything that makes it possible to really understand the interface by just looking at the doc.

A few possibilities:

1. Inline the contracts when can be. That would make the above be documented as:

   foo

   • foo | { _ : String }

   This is nicer in that example, but probably not always what we want (and might even lead to infinite recursion for cyclic contracts)

2. Add hyperlinks to the documentation (and also document somewhere the non-exported fields that are needed). This could be rendered as:

   foo

   • foo | TypeOfFoo

   Non-exported fields:

   TypeOfFoo

   { _ : String }

   (where TypeOfFoo in the documentation of foo is a link to ## TypeOfFoo).

3. Add hyperlinks, but only if the value is exported. So the example would render exactly as today, but

   {
   TypeOfFoo = { _: String },
   foo | TypeOfFoo
   }
   

   would render as

   foo

   • foo | TypeOfFoo

   TypeOfFoo

   { _ : String }