Document the WithRemoveContinuedAsNewInstances backend option

[Copilot]

Added documentation for the WithRemoveContinuedAsNewInstances backend option that was missing from the backends documentation.

This option controls whether workflow instances that complete using ContinueAsNew are immediately removed (including their history) or retained according to the configured retention policy. The option is particularly useful for workflows that frequently use ContinueAsNew to prevent storage bloat.

The documentation now includes:

• A clear description of what the option does
• A brief explanation of ContinueAsNew for users who may be unfamiliar with the concept
• Guidance on when to use this option (preventing storage bloat in frequently restarting workflows)

This brings the documentation in line with the other backend options that are already documented.

Fixes #398.

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