Preserve authored library landing page content before generated roadmaps

As a documentation maintainer,
I want all authored content in each library landing page to be preserved before generated roadmap content,
so that library introductions and explanations remain visible when the roadmap is built.

Context

Parent Epic: #125. Follow-up to #131.

The current generator replaces the body of docs/<library>/index.md, retaining only YAML front matter.
This discards authored text from published pages. This Story supersedes #131's body-replacement requirement:
the authored page is retained and build-generated content follows it.

Acceptance Criteria

• Every library-level docs/<library>/index.md retains all authored body content in its original order, including headings, paragraphs, lists, links, images, and code blocks.
• All generated roadmap content, including text, graphs, support tables, and documentation-version links, appears after the complete authored body.
• Required YAML front matter remains preserved.
• When a roadmap is missing or unpopulated, the existing default message and any generated documentation-version links follow the preserved authored content.
• Repeated generation does not duplicate authored content or generated sections, and configuration changes refresh only the generated content.
• Both website HTML and portable Markdown expose the same authored-before-generated order with working references.
• Version-level landing pages remain outside the scope of this library-level generation.

Constraints

• Keep build-time generation and the portable Markdown publication contract; introduce no MDX or raw HTML in documentation sources.
• Authored source files must not be rewritten by roadmap generation.

Definition of Done

The project's common Definition of Done applies.

• Regression tests cover rich authored content, populated and unestablished roadmaps, repeat generation, and configuration updates in both publication formats.
• Representative published landing pages are visually checked to confirm content order and readability.
• Maintainer documentation describes preservation and the placement of generated content.
• npm test, npm run build, and applicable Markdown lint checks pass.
• composer test-local passes if defined; otherwise record it as not applicable to this Node.js documentation repository.
• composer quality passes if defined; otherwise record it as not applicable to this Node.js documentation repository.