Add comprehensive landing page guidance documentation

[Copilot]

This PR adds comprehensive guidance for creating effective landing pages for Document Assembly Line interviews, addressing all requirements from the issue:

What's New

New Documentation Page: docs/authoring/landing_pages.md - A complete guide covering:

• Why landing pages matter for both users and organizations
• Essential elements every landing page should include
• Best practices for content creation and user experience
• SEO optimization strategies specific to landing pages
• Integration with project development workflow
• Common mistakes to avoid and success metrics
• Practical examples and helpful resources

Key Features

Addresses Core Requirements:

• Emphasizes sharing landing page URLs instead of direct interview links
• Focuses on just-in-time help strategies since users often don't read detailed content
• Provides specific SEO guidance including meta descriptions and user-friendly language
• Highlights essential information users need before starting interviews

Strategic Integration:

• Added to the "Authoring interviews" section for logical placement
• Cross-references existing SEO documentation without duplication
• Updates scattered landing page mentions across documentation to point to the new comprehensive guide
• Links to related resources (style guide, project management, plain language guidelines)

Files Modified

• Created: docs/authoring/landing_pages.md - New comprehensive landing page guide
• Updated: sidebars.js - Added navigation entry
• Updated: docs/get_started/project_management.md - Cross-reference to new guide
• Updated: docs/authoring/github.md - Cross-reference to new guide
• Updated: docs/style_guide/question_style_structure.md - Cross-reference to new guide

The new documentation provides actionable guidance for creating landing pages that improve user success rates while complementing existing SEO best practices documentation added in PR #511.

Fixes #245.

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