Add links to newly split Package pages

• Link to the merge request of the new page: gitlab!87446 (merged)

  • doc/development/packages.md moves to doc/development/packages/index.md and new child pages:
    • doc/development/packages/new_format_development.md
    • doc/development/packages/settings.md
    • doc/development/packages/structure.md

• Give the MR a descriptive title.

• Link to the merge request that introduced the new page.