Support for generating Enum description files

[reteps]

• Our project has a large number of enums, and would like to hyperlink to these enums in the schema themselves, as individual files. This would allow for:

1. A less bloated index.md file
2. Better discovery of these enums

For backwards compatibility reasons, in #727 I implemented this such that there is no enum template by default, and you can specify one if desired.