Have logo link default to current index

[story645]

Because one of the primary ways to navigate to the index is via logo, the default link for the logo being matplotlib.org/stable gets really frustrating when editing the landing page.

https://github.com/story645/mpl-sphinx-theme/blob/6d13c7b1c1ed722d63d08697cd48278a9615acc4/mpl_sphinx_theme/__init__.py#L21-L22

[dstansby]

I think being able to do this depends on pydata/pydata-sphinx-theme#1664 upstream