Document cog.File() and cog.Path()

[zeke]

Does what it says on the tin.

Part of #407

[bfirsh]

A few notes:

1. It might worth mentioning that File and Path are how you get files in and out of models. That's kind of implied, but not explicit.

2. I wonder if it's worth wrapping the example with class Predictor(BasePredictor):? That might be confusing to someone not intimately familiar with Cog.

3. It's a bit confusing how the File example uses Path as an input. I reckon File will more commonly be used as input than output. Most libraries make it easy to read from a file handle, easy to write to disk, but most don't expose a file handle. E.g. it's easy to read a file from a file handle with Pillow (Image.open(fh)) but it's not clear how to get the file handle back out of it again. (Maybe the object itself is a file handle? I'm not sure...)

4. I wonder whether we could have a more real example to illustrate what's going on? E.g.

   from PIL import Image
   
   class Predictor(BasePredictor):
       def predict(self, image: File):
            img = Image.open(image)
            ...

   (not sure if this actually works...)

[zeke]

Good feedback! Made some updates. This is ready for another look.