Add option to customise logo link in theme.conf

[dstansby]

In matplotlib we have a sphinx theme that is a thin wrapper around pydata-sphinx-theme to provide consistent configuration across multiple projects. This is implemented with a custom theme.conf file.

However not all options that can go into html_theme_options in conf.py are supported by theme.conf. Specifically we would like to set the default logo link in our theme.conf, which looking at the default theme.conf doesn't seem to be supported by pydata-sphinx-theme at the moment.