Add Change Log builder to NeMo
# Changelog
- Build a raw changelog automatically for every new NeMo release based on a comparison from PRs between newest version and previous version. 
- Raw changelog is then tidied to display on final page. 

# Usage

Call the builder and visit the GitHub Action that builds the changelog. It's "Print Changelog" section will have the raw changelog, which needs to be tidied up. 

```sh
git tag vX.YZ
git push origin vX.Y.Z
```

## Redo steps
If something should go wrong tag can be deleted using 

```sh
git tag vX.YZ -d
git push origin vX.Y.Z -d
```

