Document the contract versioning rule: increase on any change of result, independently of applications #96

Problem

#95 (closing #94) kept the operation contracts at version 1 in Sigima 1.4 and wrote in doc/contributing/guidelines.rst that "version 1 of the contracts is settled by the first stable release of an application recording them (DataLab 1.4): until then, contract versions are not increased". This ties the versioning of Sigima to the releases of an application, which is wrong: Sigima is a reference of its own.

The actual rule is:

• the version of a contract is increased whenever a change of a computation function changes the result it returns, in the change itself, whatever the releases of DataLab or of any other application;
• keeping version 1 between 1.3.0 and 1.4.0 was a one-off exception (contracts just published, not used yet), which does not apply to later changes.

The rule is also poorly documented: only in the contributing guidelines; nothing in the user guide, nothing in the instructions for AI agents (whose "adding a function" example even omits operation_id).

Proposal

• Guidelines: state the rule, its exceptions (previously refused inputs now accepted; changes without effect on the result; changes entirely covered by a versioned rule of the execution context, whose version is increased instead) — inputs previously accepted and now refused do require an increase —, a checklist for changing a computation function, and the historical note on 1.4.
• New user guide page "Operation contracts and versions".
• Release note for 1.4 on the one-off exception.
• .github/copilot-instructions.md: operation contracts section, corrected example, checklist for modifying a computation function.
• Docstrings of sigima.proc.contracts and computation_function(contract_version=...).
• French translations.