Generate documentation from abstract template generator

This generator should also be extended to replace the following documentation generators:

- https://github.com/openactive/developer-documentation/tree/master/generator (used for https://developer.openactive.io/data-model/types/)
- https://github.com/openactive/extension-documentation-generator (used for https://www.openactive.io/ns-beta/)

The refactor in #30 will make this easier