[#36] Remove the mention of `indigo-trial`

Description

Problem: The word "trial" apparently may sound intimidating because in software world it often means that someone is trying to sell something. One may think that "Indigo CLI Trial" has some limitations while "Indigo CLI" is a paid full version. So it's better to find a better word.

Solution: Remove the mention of indigo. Use indigo via docker instead.

Related issue(s)

Resolves #36 (closed)

✅ Checklist for your Merge Request

Related changes (conditional)

• Dependencies

  • We depend only on package versions that have been released on hackage (i.e., we don't fetch dependencies from git repositories). Failing that, we depend only on commits from the project's master branch, and not from any other branch.

• 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.