Using `index.rst` instead of `Index.rst` makes deployed docs inaccessible

[eliashaeussler]

I accidentally named the docs entrypoint index.rst instead of Index.rst.

Once deployed, this resulted in a redirect from https://docs.typo3.org/p/<package>/<version>/en-us/ to https://docs.typo3.org/p/<package>/Index.html, which is of course not available and responds with a 404 Not Found error.

@linawolf confirmed that the docs entrypoint must be named Index.rst (capital I), which is of course not a problem. But it would be nice if there's any hint during rendering, in case the entrypoint differs from the expected naming. Another option would be to allow both index.rst and Index.rst.