[TM-302] Document how fees work in practice

Description

Problem: Transaction fees and storage costs are not described in enough detail, which serves as a source of confusion.

Solution: Add a section about costs to michelsonExploration.md

Related issue(s)

https://issues.serokell.io/issue/TM-302

✅ 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.

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    • README
    • Haddock
    • docs/

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