docs: remove changelog documentation (#2540)

#### What type of PR is this?

/kind documentation

#### What this PR does / why we need it:

目前 Halo 处于多版本同时并存发行的现状，所以在各个分支（main、release-1.5、release-1.6）维护 CHANGELOG.md 会变得比较麻烦。所以考虑移除掉仓库内的 CHANGELOG.md 文件，后续以 [releases](https://github.com/halo-dev/halo/releases) 页面提供。

Ref #2538

#### Which issue(s) this PR fixes:

Fixes #2538

#### Special notes for your reviewer:

/cc @halo-dev/sig-halo 

#### Does this PR introduce a user-facing change?

```release-note
None
```