Redirect index files in proxito instead of serving
Redirect index files in proxito instead of serving

This lets us just return a redirect if the file exists.
This matches the existing nginx behavior,
except it actually appends the index filename instead of just a slash.

I'm not sure which is better,
but this is more explicit which feels good.

rtfd/readthedocs.org,rtfd/readthedocs.org,rtfd/readthedocs.org,rtfd/readthedocs.org