Improve Upgradeability Contract Examples

[HCastano]

The current examples of upgradeability and call forwarding are confusing, incomplete, and
and some cases misleading.

We should update these examples to show correct behaviour, and to guide contract
developers into using correct patterns.

List of examples that we should aim to provide:

• A basic proxy with delegate_call
• A EIP-1967 based proxy

Concepts we want to illustrate:

• Storage collisions and corruption
• Safe delegate calls
• Effects and implications of different ink::env::CallFlags

### Tasks
- [x] `delegate_call` example
- [x] ~~A EIP-1967 based proxy~~ Achieved by `set_code_hash()`

[DoubleOTheven]

This project in ink!ubator can elp here. They reference this issue in their application here.

[SkymanOne]

I'll move this to ink docs repo as we just need to document this properly