docs: correct Python transaction and value examples

[tclinkenbeard-oai]

Summary

The Python binding does not provide the documented db.transaction() context manager, and transaction reads return Value futures even when a key is missing.

• Remove the unsupported context-manager example and point readers to create_transaction() for explicit commits and retries.
• Use Value.present() for presence checks, including stored empty byte strings, and resolve the counter read before checking or unpacking it.

Validation

• Strict MkDocs builds passed for FoundationDB 7.1, 7.3, and 7.4.
• Checked the documented APIs against the upstream 7.1.67, 7.3.77, and 7.4.6 release sources.

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