Add gfm output format for changelog render

[lcawl]

Relates to #3057

AI implementation summary

• ✅ Extended ChangelogFileType Enum - Added Gfm with proper attributes
• ✅ Updated CLI Parsing - Accept --file-type gfm option
• ✅ Created GFM Renderer - Single-file markdown with clean headings
• ✅ Updated Main Dispatcher - Route GFM requests properly
• ✅ Updated Documentation - CLI reference and examples
• ✅ Added Comprehensive Tests - Full test coverage

🚀 Usage

Users can now generate GitHub-ready changelog markdown with:

docs-builder changelog render \
  --input "./bundles/1.10.0.yaml" \
  --file-type gfm \
  --output ./github-release

This produces a clean changelog.md file perfect for copying into GitHub releases.

🔧 Key Improvements Over Standard Markdown

1. Single File: One changelog.md instead of multiple files
2. Clean Headings: ### Features and enhancements (no anchor brackets)
3. GitHub Optimized: Perfect formatting for release notes
4. Complete Integration: Works with all existing options (--dropdowns, --hide-features, etc.)

The implementation is minimal, follows established patterns, maintains backward compatibility, and includes comprehensive test coverage. Teams can now easily generate clean markdown for their GitHub releases without the docs-builder specific formatting.

Steps for testing

I tested this command as follows:

1. Refresh docs-builder for local testing per https://github.com/elastic/docs-builder#building-locally:

   ./build.sh clean
   ./build.sh publishbinaries

2. Run the "changelog render" command against your release bundles. For example, in the kibana repo I ran:

    /path/to/GitHub/docs-builder/.artifacts/publish/docs-builder/release/docs-builder changelog render \
    --input "/path/to/GitHub/kibana/docs/releases/kibana/9.3.0.yaml" \
    --file-type gfm \
    --output ./docs/release-notes/

The command generates a changelog.md file that contains content like this:

## 9.3.0

### Features and enhancements
* Support subqueries in AST. [#241227](https://github.com/elastic/kibana/pull/241227)
* Supports chain controls. [#242909](https://github.com/elastic/kibana/pull/242909) [#238473](https://github.com/elastic/kibana/issues/238473)
* Add subqueries support for walker and visitor. [#241451](https://github.com/elastic/kibana/pull/241451)

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