Closes#533
Folder meta.json files listed "index" explicitly in the pages array,
causing fumadocs to include the index page twice in the flat navigation
list — once as the folder index and once as a child page. This made the
"Next" link on index/overview pages point back to themselves instead of
the first child page.
Fix: remove "index" from the pages array in all folder meta.json files.
Fumadocs automatically uses index.mdx as the folder index page without
needing it listed in pages.
