docs(python-sdk): document replace_existing_data flag and multi-context send_to_cloud

[GuyEshdat]

Summary

• Documents the new replace_existing_data constructor parameter on ElementaryCloudClient, including the False (merge) vs True (full-replace) behavior and the ~1 month TTL for assets/tests not seen in merge mode.
• Adds a new "Sending results for multiple assets" section explaining the one-context-per-asset pattern, with a list-based send_to_cloud example and a note on deduplication behavior.
• Updates Best Practices to reflect the recommended "one send call per project run" pattern.

Tracks SDK changes in elementary-python-sdk#16.

🤖 Authored by Claude (Claude Code).

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