Revert "DOC: Spihnx theme, don't sticky nav on api ref page, toc too long to fit on screen"

This reverts commit 3e618c8e3c78eec87dc84a4d745ed986331cf711.
Revert "DOC: modify theme css to keep navigation fixed on screen"

This reverts commit a625fbde59ad262efd142244ac39981c6e7d5de8.