Fixing header height

[choldgraf]

I realized that our header height was not properly being set, and this was causing h1 and h2 titles to be too-close to the header when clicking anchor links.

The problem was that we defined a --pst-header-height variable, but we did not force the header height to max-out at that variable. So the content of the header was causing it to expand in height. This made it look correct to us, but technically the header height wasn't the variable we had set. Because the anchors were set to a padding relative to the header height variable, their positioning was wrong.

So this PR forces the header to have the height we want, and increases the height from 3rem to 4rem which mimics the behavior we are used to. It also adds vertical padding to the navbar brand div so that it doesn't take too much space.

Old behavior:

New behavior:

[12rambau]

+1 for me. I checked a nearly empty page to verify that the footer was still visible and it didn't changed anything in this one so that's perfect: https://pydata-sphinx-theme--1102.org.readthedocs.build/en/1102/examples/persistent-search-field.html