[lldb][docs] Attempt to disable the generated GitHub button on the LLDB website

For unknown reasons the alabaster theme on the docs server is always generating
a GitHub link in the side bar. Beside the privacy problems of having an iframe
to some third-party service, we never configured any GitHub integration so
this button just links to the GitHub main site.

The button generation should be disabled by default, but as that's apparently
not true in the alabaster theme on the server, this patch tries working around
the issue by just explicitly turning off the GitHub integration.