docs: jinja2.exceptions.TemplateNotFound: apistar\index.html

[sdementen]

Hello,

When using both the CLI and the programmatic interface to serve or build docs from a schema, I receive a the exception jinja2.exceptions.TemplateNotFound: apistar\index.html.

using apistar==0.6.0 installed via pipenv on windows

fix is to not use os.path.join for building apistar\index.html but do theme + loader.delimiter + 'index.html' in apistar\core.py