docs(operations): use typed 2.x errors and add StepContext.attempt for Python

[ayushiahjolia]

Issue #, if available: aws/aws-durable-execution-sdk-python#663

Description of changes:
Updates the Python tabs in step.md, invoke.md, and child-context.md to match the 2.x SDK error hierarchy, and documents the attempt field on the Python StepContext.

mdformat --check docs/, codespell docs/, check_example_refs.py (529 files), and zensical build --clean all pass.

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[yaythomas]

instead of tucking this away here, I wonder if would help adding a News/Announcements section called:

Label / slug: "What's New" / whats-new
Placement: last top-level tab in the docs nav
Structure: folders per SDK

[ayushiahjolia]

For this PR I'm intentionally following the same pattern the Java language page already uses for its 2.x migration (a link-out to the SDK's migration guide from the language reference), so Python stays consistent with the existing convention here.

The "What's New" hub is a great fit for the news/announcements space we talked about in the team meetings - a place to post whenever we ship new features. I'll take that on separately.