[DOCS] Add stage recompute intro

### 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?

Thanks to the efforts of @advancedxy and @yl09099 , the stage recompute has been supported.

We'd better to add more docs to describe this design and effect, and the dedicated article is also necessary for this
big feature

### Affects Version(s)

master

### Improving the documentation

_No response_

### Anything else

_No response_

### Are you willing to submit PR?

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

The issue names no files, tests, or entry points. Start by locating the existing stage recompute design and documentation in the Uniffle repository, then document the design and its effects in a dedicated article; done means the feature has a clear, standalone introduction.

Written by the indexing model from the issue text.