Document template data schema using JsonSchema and provide some way of viewing it

[kellpossible]

If users want to customize templates using #33 they will need to understand the shape of the data being provided to the templates, and ideally they should not need to read Rust code for this.

We can generate the schema using this macro https://docs.rs/schemars/latest/schemars/ then perhaps the schema for the template can be fetched using some special header like the mime type application/schema+json

[kellpossible]

Potentially this tool can be used to view the json schema