Generate API reference documentation for alias statements

### Clear and concise description of the problem

It appears that we do not generate website reference documentation for `alias` statements, even though aliases are just as much part of the public API of a package as anything else. This issue tracks extending our documentation generator to create reference documentation for type aliases. This is especially important for templated aliases designed to be used to construct synthetic types, such as the visibility and merge-patch transform templates.

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Read the [docs](https://typespec.io/docs/).
- [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.