Reorganize documentation: move AI and Add-ons to Advanced section

[oblomov-dev]

Summary

This PR reorganizes the documentation structure by moving AI-Assisted Development and Add-ons content from the Getting Started and Resources sections into the Advanced section, creating a more logical information hierarchy.

Key Changes

• Moved AI-Assisted Development: Relocated from /get_started/ai to /advanced/agent with updated navigation links and internal references
• Moved Add-ons documentation: Relocated from /resources/addons to /advanced/addons
• Updated navigation structure:
  • Removed "AI-Assisted Development" from Getting Started sidebar
  • Added both "AI-Assisted Development" and "Add-ons" to Advanced section sidebar
  • Changed Resource section default link from /resources/addons to /resources/references
  • Removed redundant "Add-on" link from Resource section items
• Updated cross-references: Fixed all internal links throughout documentation (in cookbook pages, about page, next page, and agent page itself) to point to new locations (/advanced/addons and /advanced/agent)
• Enhanced Getting Started: Added pointer to AI-Assisted Development in the about page to guide users interested in AI-assisted development

Implementation Details

• All documentation links have been systematically updated to maintain referential integrity
• The reorganization groups advanced topics (AI, Add-ons, Downporting, etc.) together under the Advanced section
• Resources section now focuses on references, who uses abap2UI5, and release information

https://claude.ai/code/session_01EjXUiPtvY2nTvhErbQX4eu