Create JH only CHANGELOG

Problem to solve

Currently we do not have CHANGELOG for JH only changes yet.

Proposal

1. Create a CHANGELOG.md file under the jh/ folder similar to https://jihulab.com/gitlab-cn/gitlab/-/blob/main-jh/CHANGELOG.md, and ensure the CHANGELOG button in UI is pointed to this file.
2. Ensure the JH commits which contains the CHANGELOG Git trailer are included in the JH CHANGELOG.md file created in 1.

Reference link: https://docs.gitlab.com/ee/development/changelog.html

Related issue: https://gitlab.com/gitlab-org/gitlab/-/issues/362689