Unify changelog

[ltoenning]

Currently the changelog is created manually and is published at different locations (at least GitHub Release page and Discord channel).

This process should be unified and simplified.

1. Create a CHANGELOG.md in the root of this project.
2. Define a fixed structure for every release:

# swift version X.X.X (<DATE>)

## What's changed

<PLACEHOLDER>

### Features

<LIST OF NEW FEATURES>

### Fixes

<LIST OF FIXES>

## Contributors

<LIST OF CONTRIBUTORS>

3. With every PR populate the changelog
4. Display the CHANGELOG.md within the swift application (possibly using a QTextEdit)
5. Use the CHANGELOG.md for the Discord notification and GitHub Release information
6. Add a link to the latest CHANGELOG.md on the documentation homepage