port tutorials to DocC

[tayloraswift]

the steps for doing this are:

1. Move the images in examples/ to directories inside Sources/JPEG/docs.docc, as this is required in order to display the images from a DocC article

   1. This will require updating a number of file paths within the example programs in Snippets/, and the scripts in utils/, as well as in the tutorials themselves

2. Split the mega-tutorial into multiple DocC articles, with one tutorial per article

3. Replace the https://swiftinit.org/docs/swift-jpeg hyperlinks with DocC symbol links

4. Annotate the snippets in Snippets/ and replace the duplicated code blocks in the tutorials with DocC Snippet references