Clean up documentation on faking

[prescod]

There are four places that touch on it, but no top-to-bottom tutorial on the two syntaxes, how to pass arguments, how to do localization, etc.

https://snowfakery.readthedocs.io/en/feature-fake-data-docs/index.html#fake-data

https://snowfakery.readthedocs.io/en/feature-fake-data-docs/index.html#fake

https://snowfakery.readthedocs.io/en/feature-fake-data-docs/index.html#fake-and-fake

https://snowfakery.readthedocs.io/en/feature-fake-data-docs/fakedata.html