[TM-295] Add user-defined upgrades to morley

Description

Problem: upgradeableContracts.md mentions user-defined upgrades in the introduction and lists their benefits. However, the document features only an administrator-forced upgrades proposal. We need to describe user-defined upgrades as well and show how to implement them on Tezos.

Solution:

1. Add a section about user-defined upgrades, make sure the approach is applicable to Athens.
2. Provide example contracts for user-defined upgrades. Make them work in Athens using the lambda-based approach described in the doc. Provide tests to showcase the functionality and make sure it works.

Related issue(s)

https://issues.serokell.io/issue/TM-295

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