Add migration steps from mdbook or gitbook

[SoonIter]

I read some related documents of Rust and found that most of them are written by mdbook or gitbook, have old js logic, and are difficult to expand.
Developers of other programming languages can also use rspress-related features such as i18n, automatic translation, comments from internet, etc... which i think it is a good user growth point. However, there will also be some challenges to rspress. I think the out-of-the-box experience is very important.

https://rust-lang.github.io/mdBook/format/summary.html

1. Provide compatible plugins or themes so that rust-related documents can be flexibly migrated to rspress.

2. add document about mdbook migration