Enhance Smart Contract Upgradability Documentation: Addressing Field Changes and Migration

[sczembor]

The current section on smart contract upgradability in the documentation contains information on adding new functions. However, it is missing any info regarding the following:

1. Adding New Fields: How can developers incorporate new fields (storage variables) into an existing smart contract after deployment?

struct contract {
   users: Vec<address>,
}

migrate to ↓

struct contract {
   users: Vec<address>,
   funds: Vec<u64>, // new field
}

2. Migrating Existing Fields: In scenarios where modifications are required to existing fields (e.g., changing data types, renaming)

struct contract {
   users: Vec<address>,
   funds: Vec<u64>,
}

migrate to ↓

struct contract {
   user_funds: Map<address, u64>
}