[#57] Update upgradeability doc

Description

Problem: upgradeableConctracts.md was added quite a while ago and hasn't been updated much since the creation. Now were refer to "Administrator-forced upgrades" section in the upgradeable contract autodoc, so it's time to update thi doc.

Solution: What was updated:

• Parameter and storage definitions in the doc to reflect the current state of the upgradeable contract.
• Entrypoint-wise section moved from appendix to the main part of doc. It was done because now we actually use entrypoint-wise approach nearly everywhere.
• Update type safe storage migration section with an actual example of upgradeable counter upgrade from V1 to V2.

Related issue(s)

Resolves #57 (closed)

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