Add model links via script

[mikeapp]

This is the result of a Python script that attempts to:

1. Link words in code blocks, like language to anchors in the model.md
2. Also link classes and properties in the "Definitions" in the same way

This needs review.

[glenrobson]

Preview link: https://preview.iiif.io/api/p4_model_links/presentation/4.0/

[mikeapp]

Also, the script can't link up values that don't have an anchor in the model document, like supplementing, commenting, or Model, that are listed in various tables. Should we try to add inline anchors for these in the various tables in the model document?