ci: add workflow to update changelog page

[cre8ivejp]

Part of bucketeer-io/bucketeer#814

The other repositories will trigger this workflow to update their changelog pages.
Every time the other repositories release a new tag, it automatically triggers this workflow.

Flow

Triggered
↓
Download the latest CHANGELOG.md
↓
Overrides the current one
↓
Create the PR
↓
Automatically Merge PR

E.g.

[netlify]

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