Add content for GitHub Package Management

### Additional content for GitHub Package Management

We should include information on GitHub Package Management within the training and the manual.
- What it is
- When to use it, and why
- Links to additional resources
- Optional activity for creating a package
- Optional activity for using a package

Additional context:
This is a part of the general maintenance needed for this resource and this training, and is also being tracked as a part of larger efforts to [add more advanced and well rounded content](https://github.com/github/services/projects/80).

cc @githubtraining/programs @githubtraining/implementation-engineers @DGpeach @dmirnic