Reduce footer margin only for desktop

[hugovk]

Fixes https://github.com/python/python-docs-theme/issues/100.

The margin above the footer is currently reduced by 2em.

Desktop

This makes sense for desktop widths (1024+ pixels). With the reduction:

Screenshot

If it was removed for desktop there's a bigger gap between the "Quick search" box and the copyright line:

Screenshot

Mobile

But on mobile (< 1024 pixels), it hides the copyright line:

Screenshot

So let's remove this margin only for desktops:

Screenshot

Demo build

https://hugovk-python-docs-theme.readthedocs.io/en/fix-footer-demo/

[hugovk]

Rebased on main to check the RTD deploy preview (https://github.com/python/python-docs-theme/issues/70 / https://github.com/python/python-docs-theme/pull/105)... success! ✅

https://python-docs-theme-previews--106.org.readthedocs.build/en/106/