remove subsubsections in CHANGELOG.md

Author: ytdHuang

CHANGELOG.md

@@ -5,25 +5,17 @@ All notable changes to [`QuantumToolbox.jl`](https://github.com/qutip/QuantumToo
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-<!-- some templates
-
-## [vA.B.C] (YYYY-MM-DD)  // New version tag and release date
-
-### Added            // new features or functionalities
-### Changed          // some changes
-### Fixed            // bug fix
-### Breaking         // some functionalities that are no longer supported
-### Deprecated       // deprecating some functionalities
-### Version changes  // change version numbers of [compat] in Project.toml
-### Miscellaneous    // other changes
+## Unreleased
 
-- some release notes. ([#PR-ID-1], [#PR-ID-2], ...)
+- *__We will start to write changelog once we have the first standard release.__*
 
--->
+## [v0.21.4] (2024-11-13)
 
-## Unreleased
+- This is just a demonstration about [`Changelog.jl`](https://github.com/JuliaDocs/Changelog.jl). ([#139], [#306])
 
-(We will start to write change log once we have the first standard release)
 
 <!-- Links generated by Changelog.jl -->
 
+[v0.21.4]: https://github.com/qutip/QuantumToolbox.jl/releases/tag/v0.21.4
+[#139]: https://github.com/qutip/QuantumToolbox.jl/issues/139
+[#306]: https://github.com/qutip/QuantumToolbox.jl/issues/306

docs/src/resources/contributing.md

@@ -72,7 +72,15 @@ This will start a local Vitepress site of documentation at `http://localhost:517
 
 ## [Update ChangeLog](@id doc-Contribute:Update-ChangeLog)
 
-The changelog is recorded in the file `CHANGELOG.md` in the repository. If you add some changes to the package and made a PR, you should also add some messages or release notes together with the related PRs or issues entries to `CHANGELOG.md`. After that, you can run the following command under the *__root directory of the repository__* you are working on:
+The changelog is written in the file `CHANGELOG.md` in the repository. If you add some changes to the package and made a PR, you should also add some messages or release notes together with the related PRs or issues entries to `CHANGELOG.md`, for example:
+
+```markdown
+- some messages to describe the changes. ([#issue-ID], [#PR-ID])
+```
+
+See also the [Change Log page](@ref Change-Log) for more examples.
+
+After that, you can run the following command under the *__root directory of the repository__* you are working on:
 
 !!! note "Requirements"
     If this is your first time running `make` command in the local repository you are working on or you just had reinstalled `Julia`, you should run `make setup` first.