[DOCS] Introduce MultiPartLocalStorageManager

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

### Search before asking

- [X] I have searched in the [issues](https://github.com/apache/incubator-uniffle/issues?q=is%3Aissue) and found no similar issues.

### Which parts of the documentation do you think need improvement?

_No response_

### Affects Version(s)

master

### Improving the documentation

_No response_

### Anything else

_No response_

### Are you willing to submit PR?

- [X] Yes I am willing to submit a PR!

The issue names MultiPartLocalStorageManager but provides no documentation location or requested content. First locate that class and the relevant documentation entry point, then clarify the expected coverage with maintainers; done is not defined until the documentation scope is specified.

Written by the indexing model from the issue text.