Added SubSectionNavEditor and used it for EntrySection.

src/components/archive/Section.tsx

src/components/editor/PropertyEditor.tsx

src/components/editor/SubSectionNavEditor.test.tsx

src/components/editor/SubSectionNavEditor.tsx

src/components/layout/Layout.tsx

src/components/navigation/NavItem.tsx

src/components/navigation/TreeNav.tsx

src/components/routing/normalizeRoute.ts

src/pages/entry/EntryArchiveNav.tsx

src/pages/entry/EntryOverview.tsx

src/pages/entry/EntrySection.tsx

src/pages/entry/EntrySubSectionTable.tsx