Add Python column links to all requirements.md tables

Added comprehensive Python SDK references for all serialization context operations:

1. **Simple Workflow Context**: Added links to Python code for client start/result and workflow input/output operations

2. **Activity Context**: Added links to activity worker code where payloads are encoded/decoded

3. **Child Workflow Context**: Added links showing how child workflow contexts are applied

4. **Nexus Operations**: Marked as 'N/A (not yet implemented)' since nexus operations are not yet in Python SDK

5. **Memos and Search Attributes**: Added links to relevant converter and instance code

6. **Command-Aware Codec**: Many operations use the _CommandAwarePayloadCodec which dynamically applies context based on the current command sequence

7. **Clarified .NET Bug Status**: Added section explaining that .NET issue #523 is still OPEN and the document shows intended (not current) .NET behavior

All Python links point to specific line ranges in the main branch for:
- client.py (client operations)
- _workflow.py (workflow worker operations)
- _workflow_instance.py (workflow instance operations)
- _activity.py (activity worker operations)
- converter.py (search attribute encoding)

Author: dandavison