Allow unnumbered chapters use in part titles

[pozix604]

Problem

Both prefix and suffix chapters are allowed in a book, which are unnumbered, but they must precede and succeed the numbered chapters, respectively. They are treated as a special case. They are also not allowed below part titles.

I would like to use mdbook to write a document with unnumbered chapters but with part titles. Removing restrictions on unnumbered chapters would enable this.

Proposed Solution

Remove special case treatment of unnumbered sections, especially not displaying after part titles.

Notes

No response

[LunarLambda]

I would also like this.

[fsm-xyz]

this pr have done this! [https://github.com/rust-lang/mdBook/pull/533/files]