Document profile-specific output_directory behavior

[lcawl]

This PR is a follow-up to elastic/docs-builder#3987

Summary

No action or workflow logic changed — --plan already supplies nested output_path once docs-builder supports profile output_directory.

Details

• changelog/README.md: omitted output now resolves under the profile’s output_directory if set, otherwise bundle.output_directory. Includes a serverless nested-folder example. The date-promotion notes say the same.
• bundle-create / bundle-fetch: output input descriptions (and their READMEs) match that resolution order.
• changelog-bundle.yml and changelog-bundle-pr.yml: reusable output input text updated the same way.

Error strings that only mention bundle.output_directory are unchanged; that is still a valid fallback. changelog-promotion-bundle.yml is unchanged — it already follows --plan.

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

2. If you answered "Yes" to the previous question, please specify the tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).

Tool(s) and model(s) used: Cursor Grok 4.6