feat(NcAppNavigation): add --app--navigation--offset exposed CSS Variable

[ShGKme]

☑️ Resolves

• ⚠️ Chained on chore(NcAppSidebar): correctly define and document exposed CSS variables #5861

• We often need to add offset to reserver space for navigation toggle button

  • Currently, apps do it manually from --app-navigation-padding + --default-clickable-area
  • Or even worth - hardcoded value

• Add an explicit variable for that

🖼️ Screenshots

🏁 Checklist

• ⛑️ Tests are included or are not applicable
• 📘 Component documentation has been extended, updated or is not applicable
• 3️⃣ Backport to next requested with a Vue 3 upgrade

[ShGKme]

Also in progress: basic NcAppTopBar component