doc: update doc for casm = true in scarb.toml (#2619)

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Closes #2472 

## Introduced changes
Add a description about the slow compilation when casm = true in
scarb.toml file

-

## Checklist

- [x] Linked relevant issue
- [x] Updated relevant documentation
- [x] Added relevant tests
- [x] Performed self-review of the code
- [x] Added changes to `CHANGELOG.md`

---------

Co-authored-by: Artur Michałek <[email protected]>

docs/src/appendix/scarb-toml.md

@@ -166,7 +166,8 @@ sierra = true
 
 #### `casm`
 
-See more about [CASM contract class generation](https://docs.swmansion.com/scarb/docs/extensions/starknet/contract-target.html#casm-contract-class-generation) in Scarb documentation.
+Enabling `casm = true` in Scarb.toml causes unnecessary overhead and should be disabled unless required by other tools. Tools like `snforge` and `sncast` recompile Sierra to CASM separately, resulting in redundant processing. This duplicates CASM generation, significantly impacting performance, especially for large Sierra programs. See more about [CASM contract class generation](https://docs.swmansion.com/scarb/docs/extensions/starknet/contract-target.html#casm-contract-class-generation) in Scarb documentation.
+
 
 ```toml
 [[target.starknet-contract]]