DOC: use new logo in the sphinx theme as well
With this change (https://github.com/pandas-dev/pandas-sphinx-theme/commit/84134e0804325538918324246f449e694466929b) in the theme, that should work.

However, locally, I am running into the problem that sphinx/html doesn't seem to like the svg logo's .. For local testing I added a converted png which works, but just replacing `.png` with `.svg` makes that the html does not include the image (at least in firefox).

cc @datapythonista 

OK, that issue could be fixed with css in the theme (apparently need to explicitly set the height for the img tag in the navbar to force the scaling) Are we going to keep the docs independent from the web? My preference is to use the template of the web to build the docs, and I guess this is not needed if we do that. Well, we have https://github.com/pandas-dev/pandas/issues/28580 for that discussion. In my last comment there, I put some examples of other doc sites / websites. Interested to hear your thoughts about that. 
Let's continue that discussion there.

> I guess this is not needed if we do that.

As long as we use the sphinx theme, this is needed (so since such integration won't happen today, going to merge this to have the new logo on the dev docs short term). But this relative path at least ensures to not duplicate the actual logo source. 
But even if we re-use the layouts of the web, I am not fully sure that sphinx can work with layouts/images outside of its directory (eg with this PR, sphinx copies the image to `_static` in the build directory) 

