Add content for Codespaces

[brianamarie]

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.

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

[beardofedu]

@brianamarie , we could bring back the "change colors to the Tetris blocks" activity in the github-games repo to make a change to a project using Codespaces.