Migrate Generator Guide to guide page in handbook

[MathyouMB]

As a contributor, I want the guide for contributing new question generators to be part of the CCSS Handbook so that it's easier to discover, update, and maintain alongside other contributor documentation.

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🧠 Context

The current guide for contributing to question generators lives in the questions repo: contributing-generators.md

To centralize contributor resources, we should migrate this content into the CCSS Handbook.

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🛠️ Implementation Plan

1. Create a new handbook page

   • Suggested path: docs/guides/questions-repository/contributing-generators.md

2. Copy content from the original Markdown file

   • Preserve formatting, code blocks, and headers
   • Make small tweaks if needed to align with handbook voice/tone or navigation structure

3. Add frontmatter

   ---
   title: "Contributing Question Generators"
   ---

4. Verify internal links or references

   • Update any relative links or repo-specific references to work correctly within the handbook context

5. (Optional) Add a redirect or deprecation notice to the original file in the questions repo pointing users to the handbook

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✅ Acceptance Criteria

• A new page exists at docs/guides/questions-repository/contributing-generators.md
• The content from the original questions repo is fully migrated and properly formatted
• Frontmatter is present and the page shows up in the handbook sidebar under "Guides > Questions"
• Any necessary link or context adjustments are made
• The original file in the questions repo is either removed or updated with a link to the handbook