Add optional index sorting when MemmapIndex detects unsorted data

# Feature Request: Add Optional Index Sorting When `MemmapIndex` Detects Unsorted Data

## Description
Currently, when `MemmapIndex` (or related index types) detects that the index is **not sorted**, it does not attempt to sort it automatically.
This can be a problem for large datasets, as sorting manually requires additional scripts and repeated I/O, which is extremely time-consuming for multi-terabyte corpora.

## Proposed Feature
- When loading a `.bin` or `.memmap` index, if unsorted order is detected, add an **optional flag** (e.g., `sort_if_unsorted=True`) to automatically sort the index before proceeding.
- Sorting should be done **in-place** if possible, or to a **temporary file** if memory constraints require.
- Log a **warning** when sorting is performed, including **time taken**.

Example:
```python
index = MemmapIndex.load(path, sort_if_unsorted=True)
```

## Motivation
- For large datasets (e.g., multiple TBs), discovering an unsorted index late in a processing pipeline is costly.
- A built-in sorting option would save users from having to rerun a separate preprocessing step.
- This is especially useful in workflows where indexes are generated externally or on distributed systems where ordering is not guaranteed.