Example of LTVUnicycleController with path following

[sciencewhiz]

The frc-docs trajectory tutorial uses the RamseteCommand example, which was removed in #6494. The PR says that LTVUnicycleController is a replacement, but there isn't an example of either using it in a command program nor using it for path following. There needs to be a replacement for the RamseteCommand example using LTVUnicycleController that can be used in the frc-docs tutorial.