[TM-71] Prettify README

Description

Problem: usage instructions in README are a bit hard to read.

Solution: use nested markdown lists to make them more structured and readable.

Rendered

Related issue(s)

https://issues.serokell.io/issue/TM-71

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests

  • 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.