Add changelog render --dropdowns

[lcawl]

This PR relates to #3228

In particular, it adds the ability to turn off dropdowns in the "changelog render" output, same as in the changelog directive.

Summary

Users can now use --dropdowns to render separated types (breaking changes, deprecations, known issues, highlights) as MyST dropdowns, or omit the flag for the new default flattened bulleted list format.

AI implementation details

All functionality has been successfully implemented according to the plan:

🔹 CLI Command: Added --dropdowns parameter with false default
🔹 Service Layer: Extended with Dropdowns property throughout the rendering pipeline
🔹 All Renderers: Updated to support both dropdown and flattened modes
🔹 Documentation: Added comprehensive CLI documentation
🔹 Tests: Created full test coverage for both rendering modes

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: composer-2, claude-4-sonnet-thinking