[TM-314] Add gas costs doc

Description

Problem: Gas consumption is quite complex to grasp from just looking into the code of OCaml implementation. It is unclear how exactly the gas is consumed during contract execution, especially when multiple contracts are involved. Unclear gas consumption makes us write suboptimal contracts, often hitting the upper gas limit.

Solution: Provide a document describing the main concepts behind gas consumption in the reference implementation.

Related issue(s)

https://issues.serokell.io/issue/TM-314

✅ 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.