GitLab

* [Doc] Add Chapter 7

* [Doc] Add CN part 7, 7.1, 7.2, and 7.3

* [Doc] Add CN part 7, 7.1, 7.2, and 7.3

* [Doc] 1st round copyediting, 7, 7.1, 7.2, and 7.3, removing EN parts.

* [Doc] 2nd round copyediting.

* [Doc] 3rd round copyediting.

* [Doc] Copyedit with reviewers' comments.
Co-authored-by: Mufei Li <mufeili1996@gmail.com>