Add more detail to the Presets docs page

[martriay]

We should be more clear in our docs regarding our plans and philosophy behind presets, so users know what to expect. More specifically:

• we intend to maintain only the most basic usable version of each deployable contract
• we don't plan to support "specific" ones like ERC20Mintable which implies further customization and additional modules as Ownable or AccessControl)
• the sole exception probably being upgradeable ones
• for more complex/customized contract, they should use Wizard