[#152] Documentation for ADT constructors & fields

Description

Problem: it is not always clear what constructors and/or fields of ADTs
used in contract mean, but there is now way to document them except of
using `typeDocMdDescription`, which is not chcked by the compiler.

Solution: add type-level descriptions for constructors and fields.
Demand that those descriptions are attached only to existing
constructors and fields, via a type family.

Related issue(s)

Resolves #152 (closed)

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • Root README and other README.md files
    • Haddock
    • docs/
  • I updated changelog files of all affected packages released to Hackage if my changes are externally visible.

Stylistic guide (mandatory)

• My commits comply with the following policy.
• My code complies with the style guide.