[#25] add Indigo functions to easily add documentation items

Description

Problem: Indigo has no functionality to add documentation items (if not for doing so at the Lorentz level)

Solution: add functions to Indigo's Language module that allow adding documentation items with the same ease as Lorentz's.

NOTE: tests are not currently present and will follow TM-220' MR

Related issue(s)

Resolves #25 (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:
    • README
    • Haddock
    • docs/

Stylistic guide (mandatory)

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