docs(operations): document nesting type observability trade-offs

[lyushher]

Fixes #188

Added documentation for NestingType.NESTED and NestingType.FLAT in both parallel.md and map.md.

Changes:

• Explain how NESTED preserves child operations as separate scopes in execution history
• Explain how FLAT reduces overhead by flattening child operations into the parent operation
• Document the observability trade-offs between the two modes
• Add guidance on when each option may be preferred

[yaythomas]

hi @lyushher, thank you so much for contributing!

We can't merge this yet, because this feature is still in alpha in the TS/JS SDK, so it's not actually published yet. We only update the docs on actual release, not on alpha/preview.

So this PR will be on hold until then.

With that said, when the time comes to publish this, here also some suggested active voice wording (per contribution guide here https://github.com/aws/aws-durable-execution-docs/blob/main/authoring-guide.md#voice-and-tone) for parallel:

    - `nesting` (optional) `NestingType.NESTED` (default) or `NestingType.FLAT`.
        - With `NESTED`, the parallel operation wraps each branch in its own child context with
            its own CONTEXT operation, so every branch appears as a separate
            operation in the execution history. Use `NESTED` when you want to
            inspect each branch's operations independently.
        - With `FLAT`, the parallel operation uses a virtual context for each branch and skips
            the per-branch CONTEXT operation, so branches no longer appear as
            separate operations in the execution history. `FLAT` reduces operation
            overhead by ~30% and raises the maximum-branches ceiling. Use `FLAT`
            for high-volume workloads where per-branch visibility matters less
            than operation overhead.

For map.md, same wording with branch(es) → iteration(s) and maximum-branches → maximum-iterations.

This update also only covers TS, but not the Python/Java changes, and ideally these should go in at the same time.

Also, please check the authoring guide
https://github.com/aws/aws-durable-execution-docs/blob/main/CONTRIBUTING.md#formatting, it really helps regularize the markdowns if we run mdformat on them to prepare PR.