Delete empty unused index file

[SvenRtbg]

The "guides.rst" file is the new master document according to "conf.py", see commit f3fb39c4bc87d136817722d2ae1df7c23e6201a2

[SvenRtbg]

This will fix the broken CI status, so please merge this first before any other pull requests are considered. They may be turn to successful build state afterwards if currently they fail with a warning "document isn't included in any toctree".