docs(durable): add manifest setup

[tkkhq]

Tracking

• Related: docs(durable): align local runtime guidance #347
• Related: docs(durable): align local runtime guidance #347 (comment)

Why

• the local durable walkthrough deploys immediately after defining a decorated handler, but the CLI only discovers functions declared with kind: durable in volcano-config.yaml
• following the walkthrough without the source layout, dependency file, and manifest declaration finds no durable function to deploy

Summary

• add the Python durable function source layout and requirements.txt
• add matching volcano-config.yaml declarations before the local deploy commands
• clarify that @durable defines handler behavior while the manifest registers the function with the CLI

Verification

• git diff --check origin/main...HEAD