Frontend restructuring

[delishad21]

1. Removed questions-management/list and made it the main page for the questions-management directory (this is just cleaner i think)
2. Remove the concatenation of Question List in the sidebar in components/sidebar.tsx (not too sure why it was implemented this way but all pages included in sidebar are are now all collected under config/site.ts)
3. Implemented logic for sidebar to highlight the page that is selected. (This includes subpages e.g. questions-management/edit will still cause Question Management to be highlighted)
4. Moved all current pages into the app/(default) folder. (This has no effect on the URL, it just allows for multiple root layouts) - Can read on NextJS Route Groups to find out more abt this
5. Created a new directory app/(landing) for landing and login page (the root layout for the "(landing)" directory does not include the navbar and sidebar)