Unify and rework/improve navItems/navTree in navGenerator

[NicoZweifel]

Unify and rework/improve navGenerator to return combined object and adjust navItems/navTree to use/return maps/indexes.

• use a map or index access instead of find and filter in nested anonymous functions (createNavNode) that take an object reference.
• combine navItems and navTree into one object in navGenerator and pageProps that supports traversing of the tree and index access of lowercase paths.
• use/update navGenerator.test.ts