Add a tutorial to the documentation

[pacejackson]

We should add a "tutorial" to baseplate.go like what we have for baseplate.py:

https://baseplate.readthedocs.io/en/stable/tutorial/index.html

[fishy]

Do you think https://github.com/reddit/baseplate.go/issues/102 can cover that?

[pacejackson]

Ya, this is sort of a sub-task of #102 I think.