Fix changelog render when title has spaces

[lcawl]

This PR fixes a problem that was encountered in https://github.com/elastic/cloud/pull/150210 where the docs-builder changelog render --title "November 2025" option made for poor folder names and poor anchors in the output files.

Summary

Updated the docs-builder changelog render command to format the --title option for folder names and anchors.

All existing tests pass.

Changes made

• Added TitleToSlug helper method (ChangelogService.cs):
  • Converts titles to slug format: lowercase, spaces replaced with dashes
  • Example: "November 30 2025" → "november-30-2025"
• Updated RenderChangelogs method:
  • Computes titleSlug from the title
  • Passes both title (display) and titleSlug (URL-friendly) to render methods
• Updated all three render methods:
  • RenderIndexMarkdown: Uses slug for folder names and anchors, original title for headers
  • RenderBreakingChangesMarkdown: Uses slug for folder names and anchors, original title for headers
  • RenderDeprecationsMarkdown: Uses slug for folder names and anchors, original title for headers

Usage

When the docs-builder changelog render command has --title "November 30 2025", the output has

• Folder: november-30-2025/
• Anchor: [elastic-release-notes-november-30-2025]
• Header: ## November 30 2025 (unchanged)

Generative AI disclosure

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

• Yes
• No

3. 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-1 agent