Fix link to repo in documentation and remove footer links

[hagenw]

At the moment we have:

but it should point to Github instead.

And it should also not show our footer links:

To achieve this, we can just copy the settings feom audb's docs/conf.py file:

html_theme_options = { 
    "display_version": True,
    "logo_only": False,
    "footer_links": False,
}
html_context = { 
    "display_github": True,
}