docs: adds ### For developers section in CHANGELOG

[laysakura]

Issue number and link

none

Describe your changes

Sometimes, we want to remark changes for our contributors in CHANGELOG.

Also, in "keep a changelog" format, we sometimes get confused whether ### Fixed should include some internal changes like refactoring.

I noted at the top of CHANGELOG that only the new ### For developers section is for developers, and others are for end-users.

Checklist before requesting a review

• I follow the Semantic Pull Requests rules (bugfix/feature)
• I specified links to related issues (must: bugfix, want: feature)
• I have performed a self-review of my code (bugfix/feature)
• I have added thorough tests (bugfix/feature)
• I have edited ## [Unreleased] section in CHANGELOG.md following keep a changelog syntax (bugfix/feature)
• I {made/will make} a related pull request for documentation repo (feature)

[codecov]

Codecov Report

Merging #154 (cbd8cac) into main (c49bdcd) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #154   +/-   ##
=======================================
  Coverage   87.80%   87.80%           
=======================================
  Files         203      203           
  Lines       12620    12620           
=======================================
  Hits        11081    11081           
  Misses       1539     1539           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c49bdcd...cbd8cac. Read the comment docs.