Refactor SlotArena iterators and enhance documentation (#22)

- Introduced dedicated iterator types (`Iter`, `IterMut`, `IntoIter`, `IterIds`) for improved clarity and performance in `SlotArena`.
- Updated documentation to provide clear examples and usage for new iterator methods.
- Enhanced method signatures in documentation to use Rust's link syntax for better navigation.
- Adjusted `SlotId` struct to derive `PartialOrd` and `Ord` for improved ordering capabilities.
- Ensured all iterator implementations maintain O(1) complexity for access patterns, aligning with performance goals.

Author: TKorr