[90] improve docs of Indigo tutorial and the modules it references

Description

Indigo tutorial has not been fully checked since recent changes and there are some small mistakes, additionally the modules it references also have some imperfections in their docs.

Here these docs have been rephrased/adjusted. Additionally some code-style fixes have been made in these modules.

Related issue(s)

Part of #90 (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.