Fix warnings during docs generation

We have two at the moment:

WARNING: Invalid configuration value found: 'language = None'. Update your configuration to a valid language code. Falling back to 'en' (English).

WARNING: html_static_path entry '_static' does not exist

If we fall back to 'en' anyway, let's just specify it. And we don't need any _static folder as we currently do not overwrite any stylesheets etc. Thus, let's just disable the entry in the config.