Add wetzel to documentation generators

Proposed in CesiumGS/wetzel#13 (comment)

Author: emackey

implementations.md

@@ -267,6 +267,7 @@ _None currently support draft-06 or later._
 - [jsonschematic](https://github.com/yanick/jsonschematic/) - Svelte-based schema viewer. Runs as a local web app. Supports draft-7.
 - [docson](https://github.com/lbovet/docson) - Javascript-based schema viewer.  Runs as a local web app. Supports draft-4.
 - [json-schema-for-humans](https://pypi.org/project/json-schema-for-humans/) - Generate HTML representation of a schema. Python-based. Supports draft-7.
+- [wetzel](https://github.com/CesiumGS/wetzel) - Generates Markdown and AsciiDoc. With some limitations, supports draft-3, draft-4, draft-7, and 2020-12.
 
 Schema Repositories
 -------------------