Add missing 63/64 rule documentation and a test (#462)

- Prominently point out the important differences of the `pallet-revive`
runtime.
- Add a test case illustrating the problem.

See also: https://github.com/paritytech/contract-issues/issues/257

---------

Signed-off-by: Cyrill Leutwiler <bigcyrill@hotmail.com>

book/src/welcome.md

@@ -2,6 +2,18 @@
 
 Hello and a warm welcome to the `revive` Solidity compiler book!
 
+> [!WARNING]
+>
+> Solidity on PVM is running on the `pallet-revive` runtime. This introduces **observable semantic differences** in comparison with the EVM.
+> 
+> Study the [differences](https://paritytech.github.io/revive/user_guide/differences.html) section carefully. **Ignoring these differences may lead to defunct contracts.**
+> 
+> Notable examples:
+> - The 63/64 gas rule isn't implemented in the pallet (introduces potential DoS vector when calling other contracts)
+> - Contract instantiation works differently (by hash instead of by code)
+> - The gas model implemented by `pallet-revive` differs from Ethereum 
+> - The heap size is fixed instead of gas-metered and there's a fixed amount of stack size (contracts working fine on EVM may trap on PVM)
+
 ## Target audience
 
 - **Solidity dApp developers** should read the [user guide](./user_guide.md). Solidity on PolkaVM introduces important differences to EVM which should be well understood.