Docs: Improve and add new parallel routes examples (#60981)

Continuing from: #60806

- [x] Update **conditional routes** example to render dashboard pages
based on user's role (rather than checking if the user is logged in)
- [x] Add **tab group** example to allow each slot to be navigated
independently.
  - [x] Expand the **modal** example to show full implementation
    - [x] Opening the modal
    - [x] Closing the modal
- [ ] Add **catch-all** slots example
    
Please merge after the diagrams PR:
vercel/front#28770

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Co-authored-by: Zack Tanner <zacktanner@gmail.com>