[docs] Document durable reconciler usage

[joeyutong]

Linked issue: #612

This draft PR depends on #614.

Purpose of change

This PR includes:

• documentation for the success-only durable reconciler contract in both Python and Java
• usage examples for reconciler / reconciler() in the workflow agent guide
• clarification that reconciler-backed durable execution does not change the existing async support matrix
• documentation that reconciler-based recovery relies on the persisted action state used for exactly-once action consistency
• an additional clarification that durable-call replay and action-level replay are different: code outside durable execution is re-executed only when recovery re-enters an action that has not been recorded as completed

Tests

Not run. This is a documentation-only change.

API

No.

Documentation

• doc-needed
• doc-not-needed
• doc-included