feat: add json-schema to docs deploy (#14)

I don't see any reason why we can't use github pages to serve our
json-schema? That gives us versioning (via mike) as well.

This popped up because I was updating the original
stac-utils/stac-geoparquet repo and noticed some of the tests reference
the json-schema.

To test:

```sh
mkdocs serve
```

Then, go to
http://127.0.0.1:8000/stac-geoparquet-spec/json-schema/metadata.json

Author: gadomski

docs/json-schema

@@ -0,0 +1 @@
+../json-schema