docs: update outdated documentation

[Copilot]

Several docs under docs/ had stale dependency versions, deprecated library references, broken links, and outdated Python patterns.

Dependency versions (docs/client/initializing.md)

• azure-core min version: 1.24.0 → 1.37.0
• azure-mgmt-core min version: 1.2.1 → 1.6.0
• Removed msrest from the dependency table — only used in legacy codepaths, not a dependency for current generated code

OpenCensus deprecation (docs/client/tracing.md, docs/migrate/readme.md)

• Removed OpenCensus section entirely (officially sunsetted, azure-core-tracing-opencensus deprecated)
• Added deprecation notice pointing to OpenTelemetry
• Updated OpenTelemetry example to use current API (SimpleSpanProcessor replaces removed SimpleExportSpanProcessor)

Async patterns (docs/client/operations.md)

• Replaced deprecated asyncio.get_event_loop() / loop.run_until_complete() with asyncio.run() across all three async examples

# Before (deprecated in Python 3.10+)
loop = asyncio.get_event_loop()
loop.run_until_complete(my_coroutine())
loop.close()

# After
asyncio.run(my_coroutine())

Broken links (docs/client/operations.md)

• BatchService.json link updated from removed Microsoft.Batch/stable/2020-09-01.12.0/ path to current Azure.Batch/preview/2024-07-01.20.0/

Typo fix (docs/developer/sdk_from_swagger_and_typespec.md)

• print.dpg_model.properties_properties_name) → print(dpg_model.properties_properties_name)

Original prompt

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This section details on the original issue you should resolve

<issue_title>update docs</issue_title>
<issue_description>Analyze whether some docs of https://github.com/Azure/autorest.python/tree/main/docs are out of date for current code. For outdated docs, we need update or delete them.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes update docs #3360

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