Document the --contract_extensions_override flag

[naftali-g]

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Jira ticket: TODO
Link to generated documentation: https://certora-certora-prover-documentation--458.com.readthedocs.build/en/458/docs/prover/cli/options.html#contract-extensions-override

[srunquist-certora]

Suggested change

	_Command line_
	```sh
	certoraRun --contract_extensions_override
	```
	_Configuration file_
	```json
	"contract_extensions_override": true
	```