Add content for Codespaces

### Add content for Codespaces

The training should cover Codespaces.
- What it is, how it's used, when and why it's used
- Information on availability and pricing (or at least links to where to find it)
- At least one optional group activity, or modification to an existing group activity, that can provide hands on experience

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